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Waves from the Big Bang
March 17, 2014 8:30 AM   Subscribe

"The detection of gravitational waves in the afterglow of the Big Bang — if confirmed — opens a new chapter in astronomy, cosmology and physics. The signature, seen by the BICEP2 radio telescope at the South Pole, packs at least three discoveries into one: It provides the most direct evidence for the existence of the waves predicted by Einstein; it is the proof of ‘cosmic inflation’ that physicists had been eagerly awaiting; and it opens a window into the unification of the fundamental forces of nature and into quantum gravity." NYT article, in depth Nature News feature.

Note the prominent caveat, "if confirmed": while the result seems robust, it is in significant conflict with an indirect measurement based on temperature maps of the cosmic background by the Planck satellite and other assumptions about the universe. Something will have to be revised. Meanwhile: "[o]n Monday, Dr. Guth’s starship came in. Radio astronomers reported that they had seen the beginning of the Big Bang, and that his hypothesis, known undramatically as inflation, looked right."
posted by RedOrGreen (59 comments total) 40 users marked this as a favorite

 
(There's a live webcast of the press conference starting at noon EDT, if that's your sort of thing. Also a FAQ.)
posted by RedOrGreen at 8:36 AM on March 17


I'm less interested in this as proof of Gravity Waves (as, for me, personally, the other experiments showing results seemed to be strong enough, but I know for capital S science we need further confirmation, so good for that), and also for Inflation (same thing - I know we really need better confirmation so that's great, but I was already sold)...

What intrigues me is how this might be used in Unification theories - how does this work on that front?
Because inflation is a quantum phenomenon and gravitational waves are part of classical physics, gravitational waves establish a link between the two, and could be the first evidence that gravity has a quantum nature just like the other forces of nature (see 'How to see quantum gravity in Big Bang traces').
Which has a link here.... I have to do some digging. Haven't really heard much of these sorts of studies and their contribution to unification. I heard of LiGO and GEO600. Never heard of BICEP2, so cool to hear about the other Gravity Wave detectors that were out there.
posted by symbioid at 8:49 AM on March 17


"This circumstance of an expanding universe is irritating." ~ A. Einstein
posted by Fizz at 8:57 AM on March 17


I guess the site for the press conference is getting slammed at the moment. I can't connect even to the cfa.harvard subdomain.
posted by codacorolla at 8:59 AM on March 17


I have a question, though. They keep calling these "gravitational waves", yet, it seems to me that these are more like reverberating echoes or shockwaves from the elasticity of space going through expansion. Now - in a sense, it is the same sort of thing - essentially gravity warping space or motion of space, but it's not like I would picture a "gravity wave" to be?

I picture a gravity wave as a body moving through spacetime, exerting a distortion via "framedrag"(? is that the right term?) upon the spacetime fabric itself.

But here, there is not external body producing this?

Or is it that as the plasma cools off, it becomes more dense, increasing mass, then stretches, which increases the length of the increase in gravity, cooling off the plasma more leading to more mass gain, increasing local gravity, spreading out in space, cooling... ????

Is that why it's a Gravity Wave? Because it's a process of cooling->mass gain->gravity increase->expansion->(thus less gravity in the spaces between the new masses)->cooling->mass.... etc etc...
posted by symbioid at 9:01 AM on March 17


Dr. GUTh? Really???
posted by tigrrrlily at 9:02 AM on March 17 [2 favorites]


Getting the math of the Universe to cancel out - "New modification to gravity may explain the cosmological constant."
...it turns out that the vacuum is anything but empty. And since it has energy, it should curve space and time. In other words, the vacuum of space should contain enough energy to curl the Universe up into a tight little ball or blow it apart so fast that no stars could ever form (it depends on whether the energy is positive or negative).

Given our current data, there's no argument over the approximate value of the cosmological constant: it is small and positive. So why doesn't the vacuum energy bend space and time? When physicists bolt the quantum vacuum energy on to general relativity, they get absurd results unless some kind of correction factor (to the tune of 10120) is carefully added to counteract the vacuum. This fine-tuning bothers people because there is simply no way to obtain these numbers naturally...
posted by kliuless at 9:06 AM on March 17


This footage of Andrei Linde (the "Father of Inflation") getting doorstopped by one of the BICEP2 researchers with the news of 5σ data is lovely.
posted by alby at 9:08 AM on March 17 [36 favorites]


Wait, did that dude say "inflation is the 'BANG' of 'big bang'"? That's not... that's not really right, though? The "Bang" happened before inflation, inflation was an epoch after the bang, right? Eh, pedanticism. meh.
posted by symbioid at 9:11 AM on March 17


Note guys that there's a terminology difference between "gravity waves" (waves caused by gravity) and "gravitational waves" (waves of gravitational radiation).
posted by sbutler at 9:12 AM on March 17 [3 favorites]


Actually, people are pretty sloppy with "gravity waves" and often use it to mean "gravitational waves". That's a lost cause, I think.

symbioid, I can't do this justice, but in general relativity, gravity is a property of space itself. The classic (but crude) analogy is to imagine a basketball and a billiard ball on a rubber sheet - the billiard ball rolls towards the basketball because the basketball distorts the rubber sheet. Gravity waves are just the stretching and contracting (typically quadrupolar) of space itself.

posted by RedOrGreen at 9:17 AM on March 17


I believe that those Gravity waves were emitted by Sandra Bullock's universe-shaking performance as a piece of space debris in orbit around George Clooney's rugged good looks.

Scientists also claim that Alfonso Cuarón is so hot right now, he has raised the temperature of the cosmic microwave background by 7 degrees.

And yet, the sun will eventually expand to become a red giant, swallowing the inner planets and all of these "jokes" - so humanity has only four billion years at most to invent the technology that could find them funny.

An impossible quest, perhaps; but if anyone can do it, Hollywood can.
posted by the quidnunc kid at 9:25 AM on March 17 [9 favorites]


Right - that's why I was confused. I guess I need too look into Gravitational Waves as distinct from Gravity Waves. Thanks...
posted by symbioid at 9:29 AM on March 17 [1 favorite]


This footage of Andrei Linde (the "Father of Inflation") getting doorstopped by one of the BICEP2 researchers with the news of 5σ data is lovely.

Yay! Let's open a bottle of champagne right next to the professor's laptop!

(At the 60 second mark)
posted by rlk at 9:29 AM on March 17


Yay! Let's open a bottle of champagne right next to the professor's laptop!

Taittinger? The occasion calls for Krug, at the very least.
posted by Zerowensboring at 9:37 AM on March 17 [2 favorites]


Here's the paper. The results are summarized in the last two figures (13 and 14). Figure 9 is where the "meat" of the measurement lies, showing the observed polarization power on different size scales (multipoles, larger numbers are smaller scales). Solid line is a no-gravity-waves model, dashed line is their best fit r=0.2 model.

What I've heard amongst people who work on CMB experiments is generally not "this is crap" or anything dismissive, but mostly along the lines of "If the signal is this big, we should be able to go measure it ourselves." So as much as this is exciting now, it'll be really great when we get another experiment that confirms it.
posted by kiltedtaco at 9:43 AM on March 17 [3 favorites]


Wait, did that dude say "inflation is the 'BANG' of 'big bang'"? That's not... that's not really right, though? The "Bang" happened before inflation, inflation was an epoch after the bang, right?
What the 'bang' is is poorly defined. I mean if I say 'the big bang' as a theory I'm usually referring to the theory that describes the evolution of the universe from at least post-inflation onwards, and maybe inflation itself, but probably not back further to any possible initial singularity. A lot of other people use it to refer specifically to the singularity itself, but I don't like that as that bit of the theory doesn't give rise to useful predictions (you can't really work with it theoretically), and all the power of the predictions of the theory come from the nature of the later evolution.
With inflation, it's not even terribly clear if anything should come before it (see 'eternal inflation'), let alone what did come before it, if it was something else.

I'd consider calling inflation the 'bang' of the big bang just a catchy soundbite, not that inflation doesn't play a really important role in our (ever less) theoretical ideas.
posted by edd at 9:46 AM on March 17 [1 favorite]


Thinking more, what would normally be called a 'no big bang' model of an expanding universe would be one that has essentially never exited the inflationary phase (our universe being such a place has been ruled out for quite some time), so there's maybe even more reason to say that calling inflation 'the bang' is not exactly standard nomenclature.
posted by edd at 9:51 AM on March 17


OK, I liked this explanation - I hope it's fairly accurate - as it makes a lot of sense, at least in some ways...

"According to inflation theory, the waves are the hypothetical quantum particles, known as gravitons, that carry gravity, magnified by the expansion of the universe to extragalactic size."

But that's NYT and I dunno how good their science reporting is.
posted by symbioid at 10:14 AM on March 17


Paging physicsmatt for further explanations please.
posted by Purposeful Grimace at 10:16 AM on March 17 [6 favorites]


"According to inflation theory, the waves are the hypothetical quantum particles, known as gravitons, that carry gravity, magnified by the expansion of the universe to extragalactic size."

Yep, this is fairly accurate. The idea is that there were tiny (quantum) gravity fluctuations during inflation, which were promptly blown up to (relatively) enormous size by the super-fast expansion that was going on during inflation, thus becoming gravitational waves. Later on, those gravitational waves messed with photons (packets of light) from the the cosmic microwave background (CMB) radiation, the oldest light in the universe. What we're seeing now is the imprint of those waves on the CMB.
posted by freelanceastro at 10:26 AM on March 17 [2 favorites]


I'd hesitate to refer to gravitons - usually what's going on with gravity puts you firmly in the wave end of wave-particle duality. It's definitely a quantum effect though.

What happens is that in pretty much all the fields present in the early universe during inflation (the inflaton for one, the gravitational field being an obvious other) are undergoing quantum fluctuations all the time, much like fields do nowadays. Usually quantum fluctuations die back down however.

One well known case where they don't is Hawking radiation, where part of the fluctuation gets 'trapped' on the wrong side of a black hole event horizon, and another part gets to escape (in very handwaving terms).

In inflation, fluctuations can get trapped in not an entirely dissimilar way (but still pretty dissimilar), as the cosmological horizons evolve in ways that put things out of causal contact with each other (which is important in inflation to solve all the problems it's known for solving). They as a result get 'frozen out'. The inflaton fluctuations that get frozen out get turned into density fluctuations in matter when the inflaton field collapses, but the gravitational fluctuations stay around as gravitational waves, just very redshifted.

This is all handwavy, and IANA theorist.

Or on preview, what freelanceastro said.
posted by edd at 10:41 AM on March 17 [1 favorite]


Wait, did that dude say "inflation is the 'BANG' of 'big bang'"? That's not... that's not really right, though? The "Bang" happened before inflation, inflation was an epoch after the bang, right?

Depends on how you define "Big Bang." If you think of the Big Bang as a point in history when everything now in our observable universe was in a super-hot and super-dense state, expanding outward, then yes, it's completely fair to say that inflation was the "bang" in the "Big Bang," because that super-hot-and-dense-expanding state was the way the universe was immediately after inflation. During inflation, the universe was expanding really fast -- and it was also really cold, and largely empty of matter and radiation as we usually think of them, because the rapid expansion caused the density of basically everything in the universe to drop like a stone. Only the density of the inflaton field (which was driving inflation) remained constant. But when inflation ended, the inflaton field decayed into a massive amount of "normal" particles, driving the heat and density of the universe through the roof. Appropriately enough, this period is known as "reheating."

If you want, you can say the Big Bang includes inflation itself, and the period before that (whatever that was, or even if that was), but that's a very different picture of the Big Bang than we usually think about: not hot, not very "bang" like. Though certainly big, to be sure. There's some vagueness here because we really don't know what was happening back then very well at all, which is exactly why this announcement is so exciting -- gravitational waves are potentially a new window on that epoch.

For more on this, check out this post by the excellent Matt Strassler, which is part of this series of posts on the early universe.
posted by freelanceastro at 11:20 AM on March 17 [1 favorite]


I bet Neil deGrasse Tyson is pissed that this came out too late for him to include it in Cosmos.
posted by languagehat at 11:36 AM on March 17 [3 favorites]


Yo' mama so fat...
posted by blue_beetle at 11:59 AM on March 17


I have three good friends who all work with Gravitational Waves one way or another. Two of them are currently heading to the pub to "celebrate" and the third is making bitter remarks on Twitter. In other words, I gather this is huge but my brain isn't big enough to understand why.
posted by kariebookish at 12:03 PM on March 17 [1 favorite]


Will be interesting to see whether this provides any fodder for testing the amplituhedron theory.
posted by BillW at 12:17 PM on March 17


BillW: I don't think it will. It's a first broad detection of a physical phenomenon that, while anticipated, is outside the standard model of particle physics anyway, and from what little I understand of the amplituhedron idea it's more a simplyfing calculation tool than something that would make predictions of an inflation field, least of all to an extent testable by BICEP2.
posted by edd at 12:23 PM on March 17


So today is another busy day for me; I (attempted) to watch the technical talk at the Institute for Advanced Study, though the live-stream from Harvard was slammed. They need to have a secret internet for the physicists to use so we can actually see these things. The room was entertaining as hell, people sharing active links, emailing pdfs around and generally trying to get any useful information out of the slashdotted sites. Much of the actual information I got was from twitter and a Facebook group that was being led by many of the big names of the field. I gather that #BICEP2 was the highest trending topic on twitter for a while there. Yay for that most social of sciences: physics.

So I'm a theoretical physicist, but one of the things I generally don't work on is inflationary physics. I have the minimal knowledge set, but not much depth. So I apologize to anyone who does know what they are talking about here, but I'll do my best to translate. I would suggest reading Sean Carroll's blog here (http://www.preposterousuniverse.com/blog/2014/03/16/gravitational-waves-in-the-cosmic-microwave-background/) and here (http://www.preposterousuniverse.com/blog/2014/03/16/bicep2-updates/) for commentary, as well as Matt Strassler's blog (http://profmattstrassler.com/2014/03/17/bicep2-new-evidence-of-cosmic-inflation/).

I have given a brief history of the Universe's expansion previously on metafilter here (http://www.metafilter.com/131992/Possibly-the-end-of-The-Big-Bang-Theory-Not-the-TV-show#5192598). But I know no one reads the links, so I'll recap. When we look around the Universe today, we see light from increasingly more distant objects - stars, galaxies, and collections of dust and gas. When we look at the spectrum of these things - the colors of the light coming out - we see "spectral lines," either emission lines from specific atomic transitions as excited electrons in atoms drop lines to their ground states, or "gaps" in the light from absorption lines when electrons absorb a passing photon and jump up in energy. We can see these spectral lines from atoms here on Earth; the spectrum of sodium, for example, has two strong lines in yellow light, which is why sodium lamps have that yellowish tinge. We discovered a new element in the Sun, helium, by it's emission lines well before we discovered it here on Earth.

Now, when we look at the spectral lines of far-off objects, we see the familiar patterns of the lines we know on Earth, but they are shifted to lower energies. This is called red-shifting, and is the photon equivalent of the Doppler effect you can hear as an ambulance passes you by. When the siren moves towards you, you get waves of sound hitting you more often then they would if you were stationary relative to the siren so the pitch moves up (blue-shift in the photon parlance), and once the siren moves away from you, the waves are stretch out and become "red-shifted." Because light is always measured as moving at the same speed (c = 3x10^8 m/s), the physics of photon Doppler effects has some differences from sound waves, but the idea is the same. The way we interpret the systematic redshifting of far-off objects in space is that the Universe is expanding: light was emitted in some long ago time, and during its travels towards us, the metric of the Universe - the thing that defines how we measure length - expanded. This causes the photon to "stretch out" and shifts it to lower energy. It effectively looks like objects far away are moving away from us, but this does not mean that we are the center of the Universe. Everyone else would see the exact same thing no matter where they stand. The usual analogy is blow up a balloon with dots painted on it: every dot sees every other dot move away from it, but the "center" of the balloon lies in a dimension other than the balloon surface. We parametrize the expansion of the Universe by the Hubble "constant" (unfortunately not actually a constant), which is H = 70 km/s/Mpc. For each megaparsec (about 3 million light years) away something is from us, we see it as receding at 70 km/s.

Now, if we run time in reverse, the metric contracts and everything gets closer together. This implies that things heat up: the Universe was warmer in the past. Now, with the physics as we understand it, eventually this means that the Universe was too warm for electrons to be bound into atomic orbitals. This means photons could not propagate long distances, as the Universe was just a soup of electrons and atomic nuclei (mostly protons, helium, and a bit of lithium and beryllium, this was before stars, so no heavier elements had been created from the stellar fusion furnaces). As the Universe expanded and cooled, there was a moment (really a brief period of time) when the Universe went from opaque to transparent. At the time, those photons were very high energy, many electron-volts of energy per photon. However, as the Universe has expanded by a factor of ~1100 since then, today they'd be very low energy due to redshifting. However, if we look for those low-energy photons, we are essentially looking for that bright "wall of light" from the first moment in the Universe where light could move freely.

Today, those photons are at microwave wavelengths (corresponding to a temperature of 2.7 kelvin). So we call this the Cosmic Microwave Background, the CMB. It is the earliest we can "see" directly back in time, as anything earlier is obscured by the plasma that existed before this time (corresponding to about 300,000 years after "the Big Bang"). These photons of the CMB is what today's announcement concern, but let me build up a bit more, since it's pretty subtle.

Take a look at the Planck satellite data for the CMB here (http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2013/03/planck_cmb/12583930-4-eng-GB/Planck_CMB.jpg). This is the "residual" map, encompassing what the sky would look like if you could see microwaves, subtracting off backgrounds from the Galaxy, and normalized so that spots hotter than 2.7 K are red and those cooler than 2.7 K are blue (at least, I hope these people plotted it that way, some people plot the temperatures in reverse, but it doesn't really matter for this pedagogical approach). Now, the interesting thing is that these fluctuations are about 1 part in 10^5. The CMB is VERY smooth, and you have to crank up the gain by huge factors in order to see any difference in the CMB across the sky. The hot spots correlate to locations in the early Universe where, due to fluctuations that I'll get to in a moment, the gas and matter was just slightly denser than average. This heated the gas, and caused the photons to be slightly warmer than the average. That slight overheating translates all the way down the line to a slighter warmer photon from that direction in the sky today, some 13.7 billion years later. Cooler spots are primordial under-densities.

Something that you should remember is that if you "moved over" a few billion light years in our Universe, the exact pattern of the CMB in the sky would be different, as you'd see a different set of over and under-densities reaching you at some moment in time. Some of those fluctuations have not had time for the photons to reach us yet, some of them have already had their photons zip past up a billion years ago, and we'll never see them again. In fact, if you move far enough over, and then look in the right direction, you'd see the primordial overdensity that led to the Local Group of galaxies in which we now live. Somewhere, some 40 billion light years away, we are a hot patch in the CMB for some alien scientist.

The fact that CMB is so nearly uniform is a bit of a problem. Right now, the photons coming from two different directions are passing each other for the first time in the history of the Universe. They have never had time to "talk" before, as the patches of sky from which they originate were so far apart in the early Universe that light could not move between them. It took 13.7 billion years for the light to pass between those two patches. But now ask yourself: if those two patches never were in causal contact before now, how did they know to start with nearly exactly the same energy density? We are seeing patches right now that should have started as their own private little "universe," and should have had generically different starting conditions.

There are other problems. The Universe today is "flat" over large scales: triangles drawn in flat space have angles summing to 180 degrees. But if the Universe was even slightly "unflat" in the early times, it would be drawn away from flatness. To be as flat as we know the Universe is today, it must have started VERY close to perfectly flat. How? (there's also a monopole problem, see my previous post). The solution to all of this is something called inflation. The idea is that, in the very very early Universe, when the Universe was much smaller than it was at the time of the CMB emission, there was some field, which we will call the "inflaton." It might be a field associated to a particle we know and love today, but we don't know that (and there are problems identifying it with any known particle, such as the Higgs), so we'll just imagine its a new field, and we'll call the inflaton, because adding "ton" to names to make them particles and fields is What We Do.

Now, we imagine that this field originates in some strange initial conditions (we don't know why, but to do its job, it must be in this weird orientation). Namely, the field is "stuck" at some very high value everywhere in the space. This can happen: the Higgs field is trapped at a non-zero value today, for example. This field value of the inflaton must have been much higher though. When in this configuration, it is energetically favorable for the metric of space-time, the "size" of the Universe, to expand. And expand fast. The inflaton causes the Universe to explode in size, growing exponentially in size over very short times.

So imagine you start with a small Universe that is very unflat, and energy density is massively unequal across it. If there is an inflaton, the massive expansion of the Universe causes that unflatness and inequality is completely erased: sure there's some place where the energy density would be massively different than it is here, but the inflaton expanded space so much between me and that point that it will take some ungodly number of quintillion of years for the light from that place to reach us. Similarly, flatness is created because if you take any curved surface and expand it by a massive factor, it looks very flat (for example, Earth looks pretty flat to us only a few feet off the ground, doesn't it?).

Now, when inflation ends, the Universe is huge and empty, and cold, because when you expand a volume by a factor of e^60 (10^26), things get pretty icy. However, there's a lot of energy stuck in that inflaton field. As the field drops down to a zero-value (as it must be today, since the Universe no longer inflates), it dumps the energy back into the "normal" fields that we see. So the hugely high temperatures we think the Universe started at, the Big Bang you get by rolling back the clock on the Universe's expansion, is not the truly fundamental state of the Universe, but in a sense the disengagement of the inflaton field as it finishes it's job. So you might say inflation is "before" the Big Bang, but more accurately it is that we should replace the infinite-density singularity we would predict without inflation with this inflationary phase. Before inflation, there may have been a singularity, or the Universe might have looked very different. We just don't know yet.

So inflation solves a bunch of problems. And it can even explain the primordial hot and cold fluctuations we see in the CMB. In quantum mechanics, a field cannot be made to stand perfectly still, there is always a zero-mode fluctuation (due to the Uncertainty Principle). Fluctuations in what is known as the "scalar" modes appear as hot or cold spots in the CMB today. If the vacuum fluctuation in some spot was higher than usual, that gets baked into that spot turning into an overdensity, and vice versa for cold spots. "Scalar" here just means there is one piece of information: namely field value which turns into energy density. The predictions of zero-mode fluctuations perfectly explain the CMB hot and cold spots, assuming inflation.

However, in addition to the inflaton field which can have zero mode fluctuations, there is at least one other field in the Universe that can have zero mode fluctuations. That field is the metric itself: space time can fluctuate. This is part of quantum gravity, and to a certain degree with can deal with quantum mechanics and gravity. The problems only start when the fluctuations are huge, and the self-energy in the metric is big. But this isn't that case; we're interested in small fluctuations that get locked into the Universe during inflation. The space-time metric is a complicated object; it has more than just a single "scalar" value. It is a tensor, so we talk about the scalar and tensor fluctuations from inflation.

This is then a discussion of propagating fluctuations in the space-time metric, and bending the space-time metric is what gravity IS. So this is gravity waves. Contrary to some mistakes in the reporting on this, what was discovered today was not the first discovery of gravitational waves; that was done in 1993 I believe, when we saw binary systems of stars and pulsars spiral in towards each other through energy lost into gravitational waves. But I digress.

So: inflation predicts gravity waves, related to the scalar fluctuations that give us over and under-densities. How do we measure that? Well, we measure the scalar fluctuations by looking at the intensity of light from the CMB. Even though the CMB was created 300,000 years after the inflationary epoch ended, it still contained information from inflation. Now, photons contain more information than intensity. It also contains "polarization." This is because light is a wave made up of electric and magnetic fields propagating perpendicular to each other, and those fields can be oriented in different directions. Any light can be divided into two different sets of polarizations relative to a particular reference direction. On Earth, it is useful to define polarizations relative to the ground. This is because light that reflects off water only has the polarization with electric fields parallel to the surface reflected, the perpendicular polarization is absorbed. So if you are out on the water, and you wear glasses that only allow perpendicular polarizations in, you will not get glare off of water. (This also means you can't see the reflections off of ice, so don't wear polarized sunglasses in the winter while driving. Black ice is dangerous ya'll.)

So, let's ask about the polarization of light from the CMB. There are two sets of polarizations of interest: E modes and B modes (annoyingly, not related to electric E and magnetic B photon field directions). Polarizations measure the difference in temperature of the CMB at the time of emission, way back in the early Universe. There is a very handy illustration of this here (http://background.uchicago.edu/~whu/intermediate/Polarization/polar1.html), and I recommend you look at it, because otherwise it's really hard to explain without reference to a figure or a video of me waving my hands a lot. E mode polarization is caused by a particular type of temperature difference that is related to the hot and cold spots; so it gives us more information about the "scalar" modes. B mode polarization looks like polarizations curling around a point in the sky. This is caused by a gravitational wave passing by that particular point in the sky at the moment of CMB decoupling. The stretching and compression of space as the wave passed causes the photons at that point to be pushed and pulled in a particular way that appears to us today as this particular "curling" pattern. Again, I must ask you to look at the figures here (http://background.uchicago.edu/~whu/intermediate/Polarization/polar4.html), as I can't do it justice in words.

So, today we measured B-mode polarization in the CMB, corresponding to photons coming from particular angular scales on the sky. This pretty much can only be caused by primordial gravitational waves, and right now we only expect those primordial gravitational waves from inflation (I am hesitant to say "this can only be from inflation," since I'm sure there are escape hatches in the theory. There nearly always are). By measuring the "tensor to scalar" ratio, we gain some small hints about the scale of the inflaton field: what energy value it was pegged at in the early Universe during this inflationary phase. From the numbers today, that is about 10^16 GeV (the LHC collides at 14 10^3 GeV). This scale is very interesting, since it is where we expect all the forces of nature other than gravity to unify: the Grand Unification Theory (GUT) scale. There is some tension with the non-observation of B-modes from the Planck satellite. They were measuring slightly different angular scales, so they don't directly contradict, but it would imply that the inflaton was doing something non-trivial. All very interesting

I have to go, we have a 4 pm discussion of these results coming up. This is tremendously exciting; I never thought we'd make this measurement, since I've been sort of assuming that the Universe hates theorists and would just have B-modes too small to be measured (and still perfectly fine with the theory), as that's the most annoying result possible. This is really big stuff, really cool, and a huge step forward in understanding the first moments of our Universe. Much remains to be done, and there is a lot of excited skepticism here today, and I hope other experiments will soon be able to see this and tell us whether this is correct or not. But if true, we just gained a giant new window into the first impossibly short moments of the birth of the Universe. Awesome.
posted by physicsmatt at 12:38 PM on March 17 [111 favorites]


This is tremendously exciting; I never thought we'd make this measurement, since I've been sort of assuming that the Universe hates theorists and would just have B-modes too small to be measured (and still perfectly fine with the theory), as that's the most annoying result possible.

That was essentially how I felt about this too. When I first heard about these results, my first thought was "wait, but we don't have to see that...we got lucky? We never get lucky!"
I'll stop pinching myself if/when Planck independently confirms this.
posted by freelanceastro at 12:47 PM on March 17 [1 favorite]


I would like to apologise now because:

1. My nonsense comment above contains a far-more-sexist summation of the film Gravity ("Sandra Bullock's universe-shaking performance as a piece of space debris in orbit around George Clooney's rugged good looks") than I intended; and

2. I am making tedious nonsense comments, but you are all talking about actual ripples in the fabric of the Universe from the dawn of time.

Sorry.
posted by the quidnunc kid at 12:58 PM on March 17 [8 favorites]


My future sister-in-law has spent the last several winters at the South Pole working on this and it's kind of surreal to see the stuff she's explained at the dinner table about inflation and CMB turn into her team essentially having the Nobel Prize on layaway.
posted by mikesch at 1:11 PM on March 17 [4 favorites]


How soon until I can have my flying car, telepathy, telekinesis, ESP, teleportation and transubstantiation? Oh, and my amplituhedronite pendant? I'm not being snarky. I just can't internetz today have fried my brains. Actually, I love that I can rely on Metafilter to (kinda, sorta) explain just about everything in terms I can (almost) understand.
posted by PigAlien at 1:14 PM on March 17 [2 favorites]


mikesch: My sympathies are with most of your future sister-in-law's teammates (and probably herself) when the Nobel yet again goes to a collaboration effort but only a maximum of three can be named (which I think it pretty daft in these days of big collaboration efforts).
posted by edd at 1:14 PM on March 17 [3 favorites]


Contrary to some mistakes in the reporting on this, what was discovered today was not the first discovery of gravitational waves; that was done in 1993 I believe, when we saw binary systems of stars and pulsars spiral in towards each other through energy lost into gravitational waves. But I digress.

I'm can't let binary neutron stars go by with just a "but I digress", so:

* 1993 Nobel prize in Physics for the 1974 discovery of a binary pulsar at Arecibo observatory. (Two neutron stars orbiting each other; one is a radio pulsar. The decay of the orbit, as shown in the figure here, is due to the radiation of gravity waves. So, indirect evidence.)

* The Double Pulsar, where both neutron stars are detected as radio pulsars, is the most beautiful laboratory for GR that we know of. Not my opinion, it's a fact. Yep.

* NANOGrav and other collaborations are using extremely stable pulsars to directly detect the passage of gravitational waves, like aLIGO on Earth or LISA/eLISA/??? in space. The waves reported here by BICEP2 are at lower frequencies (longer wavelengths) than even the nanoHertz waves that pulsar timing will detect.
posted by RedOrGreen at 1:15 PM on March 17 [3 favorites]


(Oh, and awesome comment as usual, physicsmatt.)
posted by RedOrGreen at 1:16 PM on March 17 [1 favorite]


So it really IS the year of the POLAR VORTEX...
posted by symbioid at 1:16 PM on March 17 [1 favorite]


Thanks for the explain physicsmatt; mostly understood, barely comprehended, much appreciated!
posted by monocultured at 2:03 PM on March 17


As usual, physicsmatt and others have explained things better than I probably can. One thing that struck me from the actual paper, though, is just how much of a nonissue the 'conflict' with Planck results seems to be.

Basically, Planck set an upper limit on the possible impact of gravitational waves that was only strong if they assume that the dependence of the amount of quantum fluctuations on the wavelength of those fluctuations was simple (i.e., the amplitude is a fixed power of the wavelength). However, most theories of inflation predict that there should be deviations from this simple, 'power law' model at some level; if you allow an additional free parameter to describe that dependence, the Planck and BICEP results are both quite consistent with each other. The strength of this free parameter comes out somewhat larger (but statistically consistent with) expected values for simple inflation models (e.g. from this paper).

Short version: the Planck data released so far really only had something significant to say on gravitational waves if you make simplifying assumptions that we wouldn't expect necessarily to be correct.

Regardless, there are a number of experiments now underway that should be able to detect a signal of this level -- the story may evolve a lot over the next year. These measurements are extraordinarily difficult, since the polarization signal is very weak. Planck hasn't released their polarization results yet, and everyone I've talked to in the past just figured they've encountered trouble getting a clean signal from their data. It's possible, though, that they've been bogged down trying to get a large (and highly bureaucratic) collaboration to agree that they've seen a real signal of this unexpectedly large amplitude, as opposed to making a mistake somewhere.
posted by janewman at 2:16 PM on March 17


One way I like to understand this is as follow:

1) Looking further into space amounts to looking further back into time because the speed of light is finite.
2) We can see as far back as 380,000 years after the "Big Bang" because that's when the universe became transparent, when nuclei and electrons combined, an event seen as the Cosmic Microwave Background (CMB).
3) There are things we suspect happened before the "Big Bang" that would explain why space is so smooth even at very large scale, one of the most popular one being the theory of Inflation.
4) But we can't see any "visible" evidence of it because the CMB is in the way.
5) However, light is polarised and polarisation is affected by gravitational waves.
6) Inflation theory predicts gravitational waves and how they're distributed.

What the BICEP2 scientists measured is just that, gravitational waves twisting the CMB polarisation, which was by no mean easy to achieve.

The "Big Bang" event is usually understood as the very beginning of space and time, as well as when known particles were created. What the theory of Inflation says is the universe doubled size 60 times over or so over an extremely short period of time, and this happened before particles were created. They were at the end of the inflationary period, where the energy of the inflation, which stopped, was "converted" into radiation and matter.

This doesn't say how time and space began. However where it's successful is how it explains in a very elegant manner many properties of the universe as we observe it today.

Once the results are confirmed, I bet you $5 that the Nobel Prize in Physics will be attributed to Alan Guth and Andrei Linde. (Physics Nobel, unlike Peace Nobel, is always attributed to individuals rather than collaborative efforts or organisations)
posted by surrendering monkey at 2:27 PM on March 17 [3 favorites]


This footage of Andrei Linde (the "Father of Inflation") getting doorstopped by one of the BICEP2 researchers with the news of 5σ data is lovely.

Paraphrased: "I always hoped it wasn't something I believed simply because it was beautiful," and the added caveat of, "I hope it doesn't turn out to be a 'trick'" warms the cockles of my heart. And makes my inner misanthrope shrink, abashed.
posted by Drastic at 2:35 PM on March 17 [5 favorites]


From alby's video (This footage of Andrei Linde (the "Father of Inflation") getting doorstopped )

The key to great science:

'I always live with this feeling: "what if I'm tricked, what if I believe in this just because it's beautiful"'

(-- oh, hi, Drastic. Good timing.)
posted by spbmp at 2:43 PM on March 17 [2 favorites]


Is this Andrei Linde's ticket to Stockholm?
posted by flippant at 3:42 PM on March 17 [1 favorite]


I think it will take further confirmation for that to happen, myself.
posted by edd at 3:45 PM on March 17


I'd hesitate to refer to graviton

Yeah, he sounds like a jerk.
posted by homunculus at 3:54 PM on March 17 [1 favorite]


homunculus: don't get me started on that. My name is more alliterative than Peter Parker's, and I'm a physicist, but where's my lab accident to grant me superpowers, eh? (I'm only a touch bitter)
posted by edd at 3:56 PM on March 17 [2 favorites]


But if true, we just gained a giant new window into the first impossibly short moments of the birth of the Universe.

Just to check though: the CMB that's being analyzed is electromagnetic radiation that was emitted 380,000 years after inflation began, at the time of the recombination event, which was polarized by whatever gravitational waves happened to be passing by at each point in space at the same point in time, within the spherical surface of the particle horizon that is however many megaparsecs wide now but was ~1100 times smaller back then, right? As opposed to something that happened within a handful of seconds or less of the beginning of inflation?
posted by XMLicious at 4:21 PM on March 17


XMLicious: yes.
posted by edd at 4:23 PM on March 17 [1 favorite]


This footage of Andrei Linde (the "Father of Inflation") getting doorstopped by one of the BICEP2 researchers with the news of 5σ data is lovely.

Yay! Let's open a bottle of champagne right next to the professor's laptop!


Like he ever needs to do any other work on it.

My spouse's reaction to the video: "Now we know what the opposite of 'punked' is."
posted by Etrigan at 5:06 PM on March 17


I have given a brief history of the Universe's expansion previously

"Sentences I will never utter" for $500 please Alex
posted by ook at 5:36 PM on March 17 [7 favorites]


Actually I've been thinking about this some more:
which was polarized by whatever gravitational waves happened to be passing by at each point in space at the same point in time
and I think it's more that the gravitational waves have influenced the distribution of the source matter, and it's that distribution that allows the radiation to be polarised. So it's not an effect on the radiation or the source at exactly that time, but an effect on the source distribution before then which just gets imprinted on the radiation when the light is able to free-stream as the universe becomes transparent.

Actually it can't imprint on the radiation until the universe is becoming transparent, as the relevant patterns to generate polarisation are on too large a scale for when the universe is completely opaque - emitted light from one part needs to scatter from something relatively distant to become polarised. I should probably go bug a theorist.
posted by edd at 4:54 AM on March 18


OK, so it's a step more complicated than I thought...
In the period when photons are scattering off electrons in the early universe (so when it is not transparent), passing gravitational waves change the temperature of those photons - giving them a slight redshift or blueshift depending on which way they're going compared to the gravitational wave. A gravitational wave travelling in a particular direction will apply a compression in one perpendicular direction and a stretch in another, so photons going at right angles to it could get either compressed and get hotter, or stretched and get colder.
That leads to particular patterns in the temperature of the material, and it's those temperature patterns in the source which then allow polarisation to be generated when the photons get scattered.
Then finally the universe goes transparent, and that polarisation gets locked in to the CMB.

So it's not a direct effect on the CMB photons, or a direct effect on the scattering material, but an effect on photons which affects the scattering material which polarises the photons.
posted by edd at 5:27 AM on March 18 [1 favorite]


A couple of other questions: much of the media coverage of this is mentioning how the inflationary hypothesis was originally presented in 1980 by Alan Guth &co. But, as far as I can tell the basic Big Bang concept is from much earlier in the twentieth century. So the competing models whose cases have been substantially weakened versus inflation if this BICEP2 analysis is confirmed, are they models that still involve a Big Bang event but have some different mechanism other than inflation to move the universe from its Big Bang state to the way it is now?

And also - I think physicsmatt is talking about spectral lines for the purpose of illustrating redshift above, but isn't one of the significant things about CMB radiation that it's full-spectrum radiation that fits the "black body" statistical distribution on some level but currently has maxima in the microwave region of the spectrum, rather than radiation just involving narrow spectral spikes like the emissions from specific elements?
posted by XMLicious at 6:40 AM on March 18


First question: yes. I don't know much about those other models, but one of the major ones is the ekpyrotic scenario - that link by one of the main proponents and gives indications of how it solves some of the major problems with bare-bones Big Bangs (namely the horizon problem, flatness problem, and lack of magnetic monopoles).

Second question: yes, the CMB is thermal, so you cannot measure its redshift directly from its spectrum so easily. A thermal distribution redshifted looks exactly like a thermal distribution at another temperature. You can calculate when the CMB should be emitted though - it's well-known how to calculate the opacity of a hydrogen plasma basically, so you can calculate what temperature it should be emitted at, and hence how much it has been redshifted to get to today's temperature. There's details of the calculation here for the brave.
posted by edd at 7:07 AM on March 18 [1 favorite]


How the Biggest Scientific Discovery of the Year Was Kept a Secret
posted by homunculus at 3:59 PM on March 20 [1 favorite]


Cosmologists Say Last Week’s Announcement About Gravitational Waves and Inflation May Be Wrong. The excitement has hardly died down before theorists have begun to mutter that there could be another explanation
posted by homunculus at 1:43 PM on March 24


Oh, pshaw. It's still a major discovery if it only provides novel observable-universe-wide detail about what was happening 14.0096 billion years ago instead of 14.01 billion years ago, isn't it?

In any case, that link is a nicely-written and carefully explanatory blog post.
posted by XMLicious at 1:59 PM on March 24


I think that post is wrong in one detail. Take
But when astronomers measure the universe’s rate of expansion, they quickly come up against a problem. The universe is expanding far too slowly to have grown this big in 14 billion years.
It's pretty trivial to invert the Hubble parameter to get a (handwavy) time to zero size of 14 billion years. Inflation doesn't really alter that significantly. Almost all that 14 billion years growing to the present size is post-inflation, and how big it got in the first trillionth of a second or whatever is basically irrelevant. Whether it started the size of a point, or post-inflation the size of a soccer ball doesn't matter when you're trying to get it to 40 billion light years across.
It looks like someone is trying to explain the horizon problem, and failing.

I think the community is more worried about systematics than anything else at the moment. Compare with, say, this post from Peter Coles:
Taking all this together I have to say that I stick to the point of view I took when I first saw the results. They are very interesting, but it is far too earlier to even claim that they are cosmological, let alone to start talking about providing evidence for or against particular models of the early Universe.
Even once that's settled it's going to be a long run till we start narrowing things down a lot further though, I agree - just like it's a long run till we figure out exactly what kind of various possible Higgs that thing that showed up in the LHC turns out to be.
posted by edd at 4:03 PM on March 24


In other physics news: The Large Hadron Collider May Have Found A New Form Of Matter
posted by homunculus at 1:49 PM on April 11


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