# Cosmic inflation and gravitational waves.

1. Aug 29, 2011

### RayYates

After the big bang, did the cosmic inflation of space occur as a gravitational wave?

2. Aug 29, 2011

### bapowell

I'm a little confused about your question. Can you explain a bit more about what you have in mind? Gravitational waves are perturbative disturbances of spacetime -- they occur "on top" of a background spacetime. Meanwhile, inflation was the exponential expansion of spacetime itself. It could have been caused by a special kind of energy, called vacuum energy, that dominated the universe at that time.

3. Aug 30, 2011

### RayYates

Let me refine my question.

I understand gravity wave as an alternating compression then expansion of space-time radiating outward. As inflation occurred, would the leading edge have been a compression wave in space-time (a shock wave) or would inflation be completely uniform (expanding balloon)?

Further more... Does a gravitational wave always radiate c velocity?

4. Aug 30, 2011

### bapowell

Inflation is more like the latter. It was a uniform and isotropic expansion of spacetime itself -- whether it was the whole of spacetime (like a balloon), or a local region of spacetime -- is not currently known.

A gravitational wave propagates at v = c in vacuum. I'm unsure, but I suspect that, like light, it slows down in a medium.

5. Aug 30, 2011

### WannabeNewton

Just to add on, a gravitational wave only propagates at c if it has a low amplitude, as far as we know. It is not easy to deduce the speed of the wave from a full solution to the EFEs.

6. Aug 30, 2011

### RayYates

It occurred to me that Dark Energy and Dark Matter seem to behave like enormous (galaxy sized) slow moving (relative to light) gravitational waves. Has anyone written about something like that?

7. Aug 30, 2011

### bapowell

How so?

8. Aug 31, 2011

### RayYates

Imagine a moment in time and "freeze" an enormous gravitational wave in place. A graph of space time showing space-time in 2D would like like a sine wave relative to "flat" space. At the top space-time would be stretched and at the bottom space-time would be compressed.

Gravity would be stronger at the bottom and there would be a repulsive force at the top.

Now imagine a slow (relative to light) moving wave several times wider than a galaxy. Wouldn't galaxies cluster together at the "bottom" of one of these waves?
Wouldn't the peaks act as a repulsive force?

Of course that would not explain the difference in the amount of repulsive vs attractive force we see with dark energy and matter. It just seems to me that these dark forces are acting similar to a gravitational wave.

9. Aug 31, 2011

### bapowell

Then wouldn't we see periodic fluctuations in the clustering of galaxies?

10. Aug 31, 2011

### RayYates

There has been work that shows some galaxy clusters seem to be moving together as group (dark flow http://en.wikipedia.org/wiki/Dark_flow) and that the expansion of space is not uniform.

11. Aug 31, 2011

### RayYates

Due to the enormous size, the periods would be very very long.

12. Aug 31, 2011

### bapowell

The dark flow phenomenon refers to the motions of galaxies relative to the background spacetime -- their peculiar velocities. The expansion of space itself, on sufficiently large scales, is uniform in agreement with the standard FRW cosmology. My point about your assertion, is that, if indeed gravity was sinusoidal in space and time, with a wavelength on the order of the size of a galaxy, we would see periodic changes in the strength of galaxy clustering. At first glance, it would also seem difficult to address the fact that structure in the universe is hierarchical -- there are galaxies, clusters of galaxies, and clusters of clusters...

Dark energy causes a uniform accelerated expansion of space. This results in behavior that is qualitatively and quantitatively different from what you propose. With regards to dark matter, there are several cosmological phenomena that dark matter helps explain, for example, galactic rotation curves and the early formation of galaxies.

13. Aug 31, 2011

### RayYates

Thank you for indulging me. I've being trying to visualize this for some time so if the following analogy is way off, please explain.

Now rather than a wave, envision ripples on a pond moving in every direction. There would be wave cancellation and wave amplification. On average the pond is flat, but there would be high spots and low spots. Objects riding inside and with one of these low spots would be experience increase gravity beyond their combined mass. ( a cosmic surfer if you will allow)

I envision galaxies sitting inside an enormous gravity well, keeping it from flying apart. Dark matter, as a concept, was added to explain this without know what it is.

That seems to rest on an assumption that space-time is relatively "flat" and that "something" is adding gravity.

How do we know space-time on (a cosmic scale) is flat?

14. Aug 31, 2011

### bapowell

Historically, dark matter was introduced to account for the unexpected galactic rotation velocities. But adding a bunch of dark matter to the universe has other consequences of cosmological significance. In particular, it affects the way galaxies form in the early universe, as well as the amplitudes of the acoustic peaks seen in the cosmic microwave background. These effects are actually in agreement with our latest data from the CMB and large scale structure surveys, and so they provide additional pieces of observational evidence for dark matter. Also, observations of astrophysical events like the Bullet Cluster strongly support the presence of particulate dark matter. So, yes, dark matter was introduced in a rather ad hoc way to explain an astrophysical anomaly. It has since, however, enjoyed additional strong support from a wide range of observations. In addition, particulate dark matter in the form of weakly interacting massive particles (WIMPS) can be rather easily accommodated in modest extensions of the Standard Model of particle physics. In addition, WIMPS fit snugly into early universe thermodynamics, with the correct primordial abundance needed to explain each piece of contemporary evidence.

We determine the geometry of the universe by measuring the location of the first peak in the CMB temperature spectrum. Physically, the first peak tells us the angular size of the observable universe when the CMB was generated. Together with an associated measure of "distance" to this event (provided by the Hubble Space Telescope and other methods), we can test the geometry of the universe with trigonometry.

15. Aug 31, 2011

### RayYates

Unless I've missed something I though the existence of WIMPS, and other particles like the Higgs boson was still theoretical. Unfortunately I lack the background to comment on the Standard Module.

I completely get that, but that brings me to the crux of my dilemma. These observations are take with light. When I take the metal picture of a two dimensional surface of the pond analogy into 3D space-time I come up with this question.

Would light passing through a gravitational wave would be refracted (as it travels through stretched or compressed space) possibly altering the red shift (distance) and apparent location (direction)?

Simply, is the fish I'm looking at below the ponds surface actually where it appears? If I didn't know about the water, how would I know?

16. Aug 31, 2011

### WannabeNewton

There is no repulsive force due to a gravitational wave. The effect of a + or X polarized gravitational wave passing through a ring of dust is not periodic repulsion and attraction. Gravity, as a force, can only attract anyways.

17. Aug 31, 2011

### bapowell

Well, sure! They are hypothetical -- but that's the way science works. They are theoretical proposals intended to explain observable phenomena. We honestly don't know if particulate dark matter is the correct answer, but observational evidence supports this hypothesis and our theories can sensibly accommodate it.

Distances are typically measured using "standard candles" or standard rulers, not redshifts. Common standard candles are supernovae and other astrophysical structures like the baryon acoustic oscillations seen in galaxy distributions provide a good standard ruler.

18. Aug 31, 2011

### RayYates

Is gravity then always associated with matter, or can a distortion in space-time exert gravity.

19. Aug 31, 2011

### bapowell

Gravity can be repulsive -- it depends on the source stress-energy. Vacuum energy, for example, can be shown to lead to a repulsive gravitational force.

20. Aug 31, 2011

### RayYates

Ah yes of course. Like the type 1A supernovae. so the relative brightness would not change, What about the apparent direction?

I don't want to confuse Gravity and a Gravitational wave.

My impression is that matter within the compressed phase of a gravitational wave will experience higher gravity.

Within the stretched phase, will the matter experience "less" gravity or "negative" gravity?

If the answer is "less" then space-time would seem to be essentially flat
If the answer is "negative" then space-time would be rather... different.