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stevenspray
2008-Dec-20, 12:32 AM
I've searched for information(within this forum) on the question I'm about to ask but to no avail. I did however found this (http://www.bautforum.com/astronomy/32733-questions-about-big-bang.html) thread with very useful info.

I decided to post this question after again reading a similar post from Universe Today (http://www.universetoday.com/2008/12/16/more-thoughts-and-now-math-on-what-came-before-the-big-bang/)
"But measurements from Wilkinson Microwave Anisotropy Probe (WMAP) show that the fluctuations in the Cosmic Microwave Background (CMB) –the electromagnetic radiation that permeated the universe 400,000 years after the Big Bang — are about 10% stronger on one side of the sky than on the other."

My question:
Is the CMBR not a consequence of the Big Bang itself? If not, what process caused the CMBR 400,000 AFTER the Big Bang?

Another question just popped into my mind after again reading that useful thread I found:
In 1998 it was discovered that the Universe was expanding exponentially and that it has started doing so very recently.
How do we know that this has been a very recent phenomena? (I know that in Cosmic terms, recent might have a different meaning) In which case, how long ago did the Universe started to speed up?

Thanks in advance!

Jeff Root
2008-Dec-20, 01:37 AM
Steven,

Your first question is a bit surprising because it should be very easy to
find explanations of the CMBR's origin all over the place. But I like to
explain things whenever I can, so...

The redshift of distant galaxies indicates that all widely-separated
galaxies are moving away from one another, and have been for a long
time. Extrapolating backward in time, we see that everything must
have been packed together very tightly about 14 billion years ago.
The laws of thermodynamics say that when a gas is compressed it
gets hot, and when it is decompressed it cools. So the Universe must
have been very hot 14 billion years ago, and has been cooling since.

About 90% of the atoms in the Universe are hydrogen atoms. So at
some point in the early Universe, there was a lot of very, very hot
hydrogen gas, compressed to enormous density. Observations done
on Earth show that at temperatures above 3000 kelvins, hydrogen is
ionized and opaque. Light cannot travel far through ionized hydrogen
without being absorbed and re-emitted. So it looks like a wall of light.
And that is what the CMBR looks like. It is the light that was released
by hydrogen atoms when they cooled enough to no longer be ionized,
and no longer absorbing the light hitting them. When the temperature
of the hydrogen fell below 3000 kelvins, it became transparent, and the
light emitted by the cooling hydrogen started out on a journey that
would often last many billions of years. That is the cosmic microwave
background radiation we detect now. It started coming toward us
about 380,000 years after the Big Bang.

Your second question-- You need to read about how the observation
of supernovae decay rates vary with distance. So do I. I haven't read
enough to know, but I'm suspicious that enough really distant supernovae
have been measured to say that the acceleration hasn't been going on
from the beginning.

-- Jeff, in Minneapolis

stevenspray
2008-Dec-20, 03:56 AM
Cheers Jeff.
So the CMBR is indeed a consequence of the Big Bang but the Hydrogen needed to cool down before it started emitting this radiation, which took about 400,000.
Much appreciated!

Spaceman Spiff
2008-Dec-20, 04:23 PM
Cheers Jeff.
So the CMBR is indeed a consequence of the Big Bang but the Hydrogen needed to cool down before it started emitting this radiation, which took about 400,000.
Much appreciated!

If you really want to learn about this phenomenon, try reading the many educational pages found here (http://map.gsfc.nasa.gov/) and here (http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation). The matter in the universe some 389,000 years after the big bang was of almost no consequence (oh, say to about 1 part in a billion :whistle:) to the origin of this radiation field (setting aside the temperature fluctuation spectrum observed by WMAP). Also, it wasn't due to some distant event whose light began "coming toward us about 380,000 years after the Big Bang". It is the remnant of matter/anti-matter (near total) annihilation during the very hot, high energy density early universe. This radiation field was then -- and is now -- everywhere. See also here (http://www.astro.ucla.edu/%7Ewright/BBhistory.html) and here (http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_4.html).

ToSeek
2008-Dec-22, 11:22 PM
Moved from "Astronomy" to "Q&A."