Notes on Relativity and Cosmology - Physics Department, UCSB

10.4. OBSERVATIONS AND MEASUREMENTS 305
the answers favor roughly Ω normal matter = .1, – the stuff we can see plus a
little bit more. As a result, this means that the dark matter is not made up of
normal things like protons and neutrons. By the way, physicists call such matter
‘baryonic 3 matter’ so that this fact is often quoted as Ω baryon = .1. A lot of this
may be in the form of small not-quite stars and such, but the important point
is that at least 75% of the matter in the universe really has to be stuff that is
not made up of protons and neutrons.
So, what is the dark matter then? That is an excellent question and a subject
of much debate. It may well be the case that all of this unknown dark matter
is some strange new kind of tiny particle which simply happens not to interact
with regular matter except by way of gravity. A number of ideas have been
proposed, but it is way too early to say how likely they are to be right.
10.4.5 Putting it all together
The last part of our discussion is to put all of this data together to see what the
implications are for Ω Λ and Ω matter . I will give you a handout with some graphs
showing a lot of this data. Many of these graphs (and some other stuff) come
from from a talk given by Sean Carroll. The transparencies for his talk are
available on the web at: (http://pancake.uchicago.edu/ carroll/talks/ltalk/).
You can look them up if you want to see the data before I get around to handing
it out.
These graphs show that that each of the three measurements put some kind
of constraint on the relationship between Ω matter and Ω Λ , corresponding to a
(wide) line in the Ω matter − Ω Λ plane. You can see that, taken together, the
data strongly favors a value near Ω matter = .4, Ω Λ = .6. That is, 60% of the
energy in the universe appears to be vacuum energy!
Now, what is really impressive here is that any two of the measurements would
predict this same value. The third measurement can then be thought of as a
double-check. As the physicists say, any two lines in a plane intersect somewhere,
but to get three lines to intersect at the same point you have to do something
right.
This means that the evidence for a cosmological constant is fairly strong –
we have not just one experiment that finds it, but in fact we have another
independent measurement that confirms this result. However, the individual
measurements are not all that accurate and may have unforeseen systematic
errors. So, we look forward to getting more and better data in the future to see
whether these results continue to hold up.
We are in fact expecting to get a lot more data over the next few years. Two
major satellite experiments (called ‘MAP’ and ‘PLANCK’) are going to make
very detailed measurements of the Cosmic Microwave Background which should
really tighten up the CMB constraints on Ω matter and Ω Λ . It is also hoped that
3 Protons and neutrons are examples of a class of particles that physicists call ‘baryons.’
Baryons are particles that are made up of three quarks.