CMB Radiation from Big Bang? Why not dark energy?

[ericosg]

just to interesting not to share.

http://bit.ly/g6sFMh

i posted a question there which i wish to post also here, and get some insight.

why is this CMB uniform and dark energy and/or dark matter so uniformly spread and are not directly related? i.e. why aren't we saying that the CMB is emitted from the dark energy as a form of dark radiation of which a certain amount isn't dark at all? and thus why are we stating that it emmited from the big bang since it's clearly still around and not a wave. It's growth would then have to be closely tied to the expansion of the universe itself, otherwise it would disappear and not just reduce in intensity.

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my apologies if this should have been posted under cosmology, etc.

 

[Bill_K]

Where to start. The link you posted is to a story about Roger Penrose's ideas on Conformal Cyclic Cosmology. This was in the news sometime last year, and since then the circular patterns claimed to have been seen in the CMB have been shown to be just statistical fluctuations. So Penrose's theory has no evidence to support it at present.

Secondly the CMB was not emitted from the Big Bang, It was emitted in the early universe, about 350,000 yrs after the big bang, at a time when the universe had cooled enough for protons and electrons to deionize and form hydrogen atoms, making things transparent. It consists of photons emitted from normal matter the usual way and certainly has nothing to do with "dark radiation from dark matter."

It's quite uniform because the universe was initially quite compact and experienced a period of extremely rapid expansion known as inflation.

 

[bcrowell]

... and experienced a period of extremely rapid expansion known as inflation.

Or not :-)

 

[ericosg]

Thank you for such a quick and clear response!

I'm thrilled I found a forum such as this and people like you! :-)

 

[Tanelorn]

Or not :-)

Ben do you have a different view on the inflation error? oops I mean era..

 

[bapowell]

The CMB is uniform because the energy density of the early universe was uniform. The creation of the CMB was a primordial event; dark energy -- if it exists -- came to dominate the energy density only 'recently'. These two phenomena are therefore not directly connected.

However, if the smoothness and uniformity of the primordial universe was prepared by inflation, than there is a connection between the uniformity of the CMB and the uniformity of the stuff that drove the inflationary expansion. That stuff would likely have been very much like dark energy -- a smooth energy component with negative pressure.

 

[Tanelorn]

Perhaps uniformity and smoothness at last scattering could be explained by the gas having had enough time to reach thermal equilibrium before beginning expansion?

Or perhaps the process of expansion not only cooled the universe but also had the effect of smoothing out temperature differences? Perhaps hotter areas expanded slightly faster than colder areas?

 

[bapowell]

Perhaps uniformity and smoothness at last scattering could be explained by the gas having had enough time to reach thermal equilibrium before beginning expansion?

Aha! Good question. This is a very important point regarding the uniformity of the CMB and motivates what's called the Horizon Problem. The size of the causal horizon at the time of last scattering subtended an angle of about 1 degree on the sky today; the present-day CMB sky has a volume 10^{88} times larger than a causal volume at last scattering. Of course, equilibrium could only be achieved within a single causal volume, since equilibrating processes are limited by the speed of light. And so the problem is thus: the present day horizon volume is highly uniform and isotropic, and, yet, it is comprised of myriad causally disconnected regions.

Or perhaps the process of expansion not only cooled the universe but also had the effect of smoothing out temperature differences? Perhaps hotter areas expanded slightly faster than colder areas?

This process would not solve the Horizon Process simply because the distance scales are so vast as to preclude any dynamical equilibration. Cosmic inflation was devised to address the horizon problem and does so by postulating that the early universe underwent a period of exponential expansion. A causal patch in the early universe was stretched and amplified to a scale surpassing our present day horizon volume. The field that drove inflation would have been correlated and uniform across this region; its subsequent decay products would have shared this uniformity. The radiation emitted in this decay process is the CMB -- uniform and isotropic across the present-day horizon.