Cosmic inflation and gravitational waves.

In summary: WIMPS) is a natural outcome of particle theories beyond the Standard Model. For example, the lightest supersymmetric particle is a candidate for dark matter. The LHC may soon be able to test this possibility.In summary, after the big bang, the cosmic inflation of space occurred as a uniform and isotropic expansion of spacetime itself, possibly caused by vacuum energy. Gravitational waves propagate at the speed of light and can have both attractive and repulsive effects on objects in space. The concept of dark matter was introduced to explain unexpected galactic rotation velocities, but it has since been supported by a wide range of observations and is a natural outcome of particle theories beyond the Standard Model. The presence of dark matter can also
  • #1
RayYates
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After the big bang, did the cosmic inflation of space occur as a gravitational wave?
 
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  • #2
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
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
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
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
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
RayYates said:
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?
How so?
 
  • #8
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
Then wouldn't we see periodic fluctuations in the clustering of galaxies?
 
  • #10
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
bapowell said:
Then wouldn't we see periodic fluctuations in the clustering of galaxies?

Due to the enormous size, the periods would be very very long.
 
  • #12
RayYates said:
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.
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
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
RayYates said:
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.
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.

How do we know space-time on (a cosmic scale) is flat?
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
...WIMPS fit snugly into early universe thermodynamics...

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.

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.

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
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
RayYates said:
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.
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.

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)?
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
...Gravity, as a force, can only attract anyways.

Is gravity then always associated with matter, or can a distortion in space-time exert gravity.
 
  • #19
RayYates said:
Is gravity then always associated with matter, or can a distortion in space-time exert gravity.
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
Distances are typically measured using "standard candles" or standard rulers, not redshifts.

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

There is no repulsive force due to a gravitational wave...
Gravity can be repulsive -- it depends on the source stress-energy.

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.
 
  • #21
bapowell said:
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.

When in the context of GR it is always better not to talk about gravity as a force (I know it just slips out but it still can be misleading) even if the attractive or repulsive force pictures borrowed from EM are very graphic they are simply not correct in general relativity.
So if we stick to the appropriate modelling of gravity as geometry, yes, a stress-energy tensor source such as vacuum has negative diagonal pressure components.
 
  • #22
TrickyDicky said:
When in the context of GR it is always better not to talk about gravity as a force (I know it just slips out but it still can be misleading) even if the attractive or repulsive force pictures borrowed from EM are very graphic they are simply not correct in general relativity.
So if we stick to the appropriate modelling of gravity as geometry, yes, a stress-energy tensor source such as vacuum has negative diagonal pressure components.
Indeed. I have in mind the Newtonian potential as a perturbation about FRW. Sorry for the poor language.
 
  • #23
Thank you Drs. I think you answered my question but just to be completely clear...

Within the stretched phase of a gravitation wave, will the matter experience "less" gravity or "negative" gravity?
 
  • #24
RayYates said:
Within the stretched phase, will the matter experience "less" gravity or "negative" gravity?
The tidal force that the + (and x polarized once you do a 45 degree rotation) polarized wave will exert on the ring of dust will operate in a manner such that, if the wave happened to be oriented along the z axis, the rate of change of the x component of the tidal force will be opposite to that of the y component. I would say this is "less" gravity, in your sense of the word, not "negative" gravity.
 
  • #25
Thanks everyone. I see the error in my reasoning.
 
  • #26
bapowell said:
... our latest data from the CMB and large scale structure surveys ... the first peak in the CMB temperature spectrum. ...

Bapowelll, Could you recommend some reading material on this? I am very interested (in this and what I assume is the related blackbody signature of the CMB), but other than general statements about the results, I can't find any / many discussions about the background / logic.

(I'm not an academic, so if the recommendation could focus on discussion rather than mathametical formuls, I'd really appreciate it.)

Regards,

Noel.
 
  • #27
Noel, that's a tough one. I don't know of any published popular accounts of the CMB accessible to non-experts, although of course there must be some. Have you tried wikipedia and other internet searches? Wayne Hu's website is excellent -- he has several CMB tutorials with illustrations and animations at a variety of levels: http://background.uchicago.edu/~whu/" [Broken]

Maybe start there. If you have questions, of course feel free to ask them here at PF!
 
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  • #28
Thanks bapowell.
 

1. What is cosmic inflation?

Cosmic inflation is a theory that explains the rapid expansion of the universe in the first fraction of a second after the Big Bang. It suggests that the universe underwent a period of exponential expansion, causing it to grow from a subatomic size to the size of a grapefruit in just a fraction of a second.

2. What evidence supports the theory of cosmic inflation?

One of the main pieces of evidence for cosmic inflation is the observation of the cosmic microwave background radiation. This radiation is the leftover heat from the Big Bang and is nearly uniform across the entire observable universe, suggesting a period of rapid expansion. Additionally, the patterns observed in the cosmic microwave background support the idea of inflation.

3. How do gravitational waves relate to cosmic inflation?

Gravitational waves are ripples in the fabric of spacetime caused by the acceleration of massive objects. Inflation theory predicts that the rapid expansion of the universe would have created gravitational waves, which have been observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO). These observed gravitational waves support the theory of cosmic inflation.

4. What is the significance of detecting gravitational waves from the early universe?

The detection of gravitational waves from the early universe supports the theory of cosmic inflation and provides valuable information about the early moments of the universe. It also opens up new avenues for studying the universe and allows us to test other theories, such as the theory of general relativity.

5. How does the discovery of gravitational waves impact our understanding of the universe?

The discovery of gravitational waves has revolutionized our understanding of the universe. It has confirmed the existence of black holes and provided evidence for the theory of cosmic inflation. It also allows us to study the universe in a new way and may lead to further groundbreaking discoveries in the future.

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