MadSci Network: Astronomy

Subject: Why does the universe need dark matter?

Date: Mon Feb 21 12:17:42 2000
Posted by Justin
Grade level: 10-12 School: No school entered.
City: No city entered. State/Province: No state entered. Country: No country entered.
Area of science: Astronomy
ID: 951157062.As

Before i start, id like to say i love your website, and have spent hours 
here reading. Now for the question

We can estimate the total baryonic matter of the universe by studying Big 
Bang nucleosynthesis. This is done by connecting the observed He/H ratio 
of the Universe today to the amount of baryonic matter present during the 
early hot phase when most of the helium was produced. Once the temperature 
of the Universe dropped below the neutron-proton mass difference, neutrons 
began decaying into protons. If the early baryon density was low, then it 
was hard for a proton to find a neutron with which to make helium before 
too many of the neutrons decayed away to account for the amount of helium 
we see today. So by measuring the He/H ratio today, we can estimate the 
necessary baryon density shortly after the Big Bang, and, consequently, 
the total number of baryons today.  
               So keeping this in mind, how much baryonic matter do we 
need to account for the known amount of light isotopes?

Re: Why does the universe need dark matter?

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