Um..."[Gravity waves] One of the greatest discoveries in the history of science." Comments, anybody?

Factual Questions
21

Confirmatory data from other experiments will be important, as it’s a difficult measurement to make. But assuming it holds up to scrutiny, it’s very exciting.

Regular followers of physics will certainly have come across pictures of the cosmic microwave background (CMB) radiation. The linked image shows a map of the sky obtained in the microwave band. Each point on the map represents the temperature (which is related to the energy spectrum) of the microwaves in that direction. The temperatures aren’t directly shown, though, as they are nearly rock-solid the same in every direction. Rather, the map shows the tiny variations around the average temperature, after a variety of less interesting effects have been subtracted out.

The colors on that map are clearly not random. There are clumps and patterns that exist over a variety of angular scales. (“Angular” is appropriate here since the map is a projection of the spherical sky.) These clumps and patterns, these correlations, are imprints of density variations present in the universe about 380,000 years after the Big Bang. The concept of “inflation” gives an attractive way to seed these density variations. With inflation, quantum fluctuations present very early on (roughly 10[sup]-36[/sup] s from the starting gun) get stretched out across the universe as it expands exponentially in the first tiny fraction of a second. These fluctuations result in subtle density variations which, in the fullness of time, result in the obvious density variations we see today (like galaxies).

This story works and gives predictions about the isotropy and homogeneity of the universe, the flatness of spacetime on large scales, the correlations in the CMB sky map, the density of dark matter, the abundance of the light elements, and more. Observations are all consistent with these predictions of inflation. So, what’s new?

Well, inflation isn’t at cut-and-dry as it might sound, and it also isn’t the only way to get the universe we see. So, it still needs to be firmed up. One stark prediction of the inflation model relates to gravitational waves. The energy scales involved in inflation are huge, like 10[sup]12[/sup] times higher than what we can access at the LHC. With such energy rippling around, gravitational waves should be produced. It is important to note here that the most straightforward interpretation of this statement is a quantum one. Quantum gravity should manifest itself. That has folks excited.

Anyway, these primordial (and potentially quantum) gravitational waves would be present through the inflationary period and would remain as the universe cooled. They would imprint signatures on the CMB when it formed 380,000 years later.[sup]1[/sup] The imprinted signature that is relevant here is something called B-mode polarization. The microwaves of the CMB are polarized a bit, and any polarization pattern can be broken down into diverging/converging patterns on the one hand (“E-mode”) and swirly patterns (“B-mode”) on the other. Gravity can give rise to B-mode polarization in the CMB.

B-mode polarization in the CMB has, in fact, been measured previously, but that polarization stems from more mundane causes and has correlations across the sky consistent with those causes. What BICEP2 has done is to measure the B-mode polarization that has angular correlations that match an inflationary origin. It is nigh impossible to get such gravitational wave patterns from anything other than quantum effects happening in an inflationary epoch at energy scales that boggle the mind.

In short, this is exciting because:

  • It reveals a messenger of things happening 10[sup]-36[/sup] seconds after the start of all things.
  • It kills off whole schools of thought on the very early universe, although inflation has been the favored picture for some time.
  • It potentially shows evidence of quantum gravity.
  • It gives quantitative access to things happening at energy scales ridiculously inaccessible to terrestrial experiments.

[sup]1[/sup]It wasn’t microwaves at the time, though. It’s been highly red-shifted since then.

22

It happens when the writer gets to 3500 words and realizes that the editor called for 4000. :slight_smile:

23

As Chronos said, the Big Bang happened everywhere. So there will always be waves from among the “first ones made long ago” that will be just reaching us at any given point in time.

24

Forget Ken Ham. I want to see how mythoughts accounts for Einstein being right. :eek:

25

Inflation is the transition of the “inflaton” (in’-fluh-tahn) field from a higher energy false ground state to a lower energy state, thereby releasing an ungodly amount of potential energy all at once. The entire inflaton field doesn’t necessarily experience this phase transition all at once, though. Pockets of the field decay and, thus, inflate up to form a universe, but in many (relatively) straightforward inflation models, this process doesn’t just cease after that one inflationary process. Rather, inflation continues eternally, only in disparate places that are separated from each other by entire universes that have inflated up. While these models make some naturalness concerns go away, they introduce others. Suffice it to say that it’s an open question.

26

Just want to note that many people above called them Gravity Waves. That’s the wrong term. They are actually Gravitational Waves. The term Gravity Wave had already been taken for a different phenomenon, so they had to come up with a different name when someone suggested waves in the gravitational field.

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If you can liste to the BBC where you are, this is worth 6 minutes - especially Neil Turok’s measured response to Stephen Hawking towards the end:

28

I like to tell people I worked on data processing for an instrument that detected gravity waves* in the mesosphere as a summer project in undergrad.

*Unfortunately they were the boring kind of gravity waves.

29

Point taken, and ignorance lessened.

But it seems sort of … churlish. And I know that you are personally, and professionally, A-OK, so don’t be mad at me :)…

You know about the following similar case, of course–or is it? Certainly an interesting comparison in terms of public interest and press prominent display. Of course Einstein was a bigger deal than these guys.

I looked it up on wiki to get the dates right, and I sort of remembered the comment by E:

The first observation of light deflection was performed by noting the change in position of stars as they passed near the Sun on the celestial sphere. The observations were performed in May 1919 by Arthur Eddington and his collaborators during a total solar eclipse,[12] so that the stars near the Sun could be observed. Observations were made simultaneously in the cities of Sobral, Ceará, Brazil and in São Tomé and Príncipe on the west coast of Africa.[13] The result was considered spectacular news and made the front page of most major newspapers. It made Einstein and his theory of general relativity world-famous. When asked by his assistant what his reaction would have been if general relativity had not been confirmed by Eddington and Dyson in 1919, Einstein famously made the quip: “Then I would feel sorry for the dear Lord. The theory is correct anyway.” [14]

30

Eddington’s eclipse observation was even more over-hyped than this one is. Many people don’t realize that Newtonian theory also predicts deflection of starlight around the Sun, just by a different amount (the Newtonian deflection is half the Einsteinian deflection). And the error bars on Eddington’s measurement were, as I understand it, wide enough to be consistent with either theory. So really, it didn’t prove anything… But it involved a lot of people with impressive titles traveling to exotic locations, so it got peoples’ attention.

31

I don’t know much about this, but I read that validation of inflation theory is considered stronger evidence for a multiverse.

So does this imply we are likely in a universe that will expand eternally, creating new multiverses as it goes (which will themselves create multiverses, whereas our universe was created the same way by a previous universe)?

How do the multiverses arise in an inflationary universe? Is it from black holes, unevenness in the universe or other things?

32

Can you expand on this a bit? What measurements found the B-mode polarization, and how did those differ from these?

Also (for anyone), is there anywhere that has the polarization image and the corresponding portion of the CMB variation image? Do they (or should they) line up at all?

33

The idea behind the inflationary multiverse is that the vast, vast majority of the multiverse is in an inflationary state, which is boring. The inflationary state is unstable, and so occasionally a piece of it will spontaneously fall into a non-inflationary state, which spreads… But the inflationary bulk spreads faster than the non-inflationary bubbles, and so even though inflation is unstable, it persists.

In this model, each of these non-inflationary bubbles is an essentially independent universe. There’s no chance of bubbles ever meeting or interacting, because the space between bubbles is inflationary, and hence expanding faster than the bubbles could ever hope to. No bubble will ever give rise to daughter universes, because only the primordial inflationary state can do that.

34

Already have it, here. Those (very bad word) CFL’s keep blowing out. I don’t care what they say about ‘longer life’ - its baloney.

35

Oh, I should also mention that Pasta is correct: These observations, and others like them, might potentially eventually give us some insight into extreme phenomena such as quantum gravity. Anything at all we could learn about quantum gravity would be big news. But that’s all just potential, and no trace of actual results exists yet.

36

The CMB is distorted by gravitational lensing from all the mass it passes on the way to being observed. Among the many, mostly subtle, effects this lensing has on the CMB pattern, one is that a small amount of the dominant E-mode polarization gets twisted into B-mode polarization. This has been seen at high confidence by the South Pole Telescope and POLARBEAR. From these measurements one can extract information about the distribution of structure along the line of sight. All these data agree well with standard Lambda-CDM cosmology.

The observational difference is in the angular scale of correlations seen in the polarization pattern. More technically, the “power spectrum” of B-mode polarization due to lensing peaks at multipoles of a thousand or so (angular scales of tenths of a degree), whereas inflation leaves B-mode power at multipoles around a hundred (angular scales of a few degrees). The left panel of this plot shows the BICEP2 B-mode power spectrum. The solid line is what you would have with lensing alone. This lensing component peaks way off to the right on the figure, which is where it was measured previously.

The maps look unrelated. This fits the inflation picture, where the polarization amplitude is driven by the graviton field and the temperature variations are driven by the inflaton field, and those fluctuations are independent. The relevant BICEP2 paper has a temperature/B-mode power spectrum plot (Figure 2, Panel “TB”) showing the predicted and observed lack of correlation.

Just to pull the “ho-hum vs. awesome” balance a bit back the other way: I would rank this news well above “no trace of actual results”. The interpretation of the signal is certainly model-dependent, but the most straightforward inflationary models, which work for all other observations, have this B-mode signal coming about via quantization of gravity. Quantitative details of such quantization may be lacking, but qualitatively they are there. If we discover, in the fullness of time, that these effects actually have a classical or non-gravitational origin, then that will be that. But if the “normal” inflationary picture holds up, then we are indeed looking at quantum gravity. And thus, within the context of these models, the empirical era of quantum gravity has started.

37

Thanks for the explanation. Could you explain the difference between gravity waves and gravitational waves?

38

Gravity waves are an everyday ho-hum phenomenon. All the term means is “wave where the restoring force is gravitational”. The waves you see on the surface of a body of water are one example of gravity waves.

Gravitational waves are waves within the gravitational field itself, ripples in the fabric of spacetime. These are extraordinarily difficult to detect, and it has not yet been done directly.

39

Two things I forgot to add…

…and the particular region of sky viewed by BICEP2 was chosen because it has very little stuff (galactic foreground) in the way.

The size of this signal is not readily predicted by inflations models in general. There was no a priori reason the signal should be this large versus a factor of 10 or 10,000 or anything smaller. This observation puts a pin on the map of what sorts of energy scales were present when inflation began.

40

In the course of futzing around on info on weak equivalence experiments–I watched that hammer and feather–which I can sort of get my head around, I happened on this nice review article on the experimental approach to the whole shebang, "GR confronts Experiment."

Section 6, beginning page 68, particularly 6.4.

From a quick scan your points are in the whole article, of course, even from my low point-of-view.

What would be most fun, in terms of this thread, to concentrate on, if that’s a reasonable question?