Hubble Diagram is probably the most traditional of cosmological test.
It plots relative luminosity distance sources as a functional derived shift.
In earlier on the brightest things that astronomers could think of to look at the
brightest cluster galaxies, they can be detected out to [UNKNOWN] from using the
technology available at that time, that there are not to be quite far enough to
really sense cosmological effects. But moreover the problem was that galaxies
had been made out of stars, evolve, because stars evolve.
And, moreover, they merge, so their total stellar mass increases.
And so by the 1980's, it became clear, that Hubble diagram for galaxies, is
basically an impossible task. Because the evolutionary effects really
dominate over any cosmological effect. The difference between different
evolutionary models is much larger than between any plausible range of
cosmological models. Supernovae were also suggested as
standard candles, as early as the 1960's by [UNKNOWN] collaborators.
But back then, nobody knew how to make them standard enough for this purpose.
The revival of this test came in mid 1990s, where astronomers learned how to
standardize brightness of supernovae in the ways that we covered earlier in the
class. And because they can be seen further away
telescopes are bigger and detectors are better, suddenly they became a viable
cosmological test again. Do note that some uncertainties still
remain about the origin of, of supernova type 1a and exactly what happens in those
explosions, but astronomers strive to come up with tests to control all
possible systematics. We do see some variation for light curves
but most of it can be then taken out through standardization procedures.
The some remaining questions such as whether there is an additional parameter,
there is an evolution and there are hints that indeed such things do happen, but at
such a subtle level they do not affect the cosmological results.
Still, keep in mind these are Messy stellar explosions which we can barely
model in computers today. And the more realistic the models would
get, the more complicated they look. So it's still possible that some
additional uncertainties might be present using supernovae standard [UNKNOWN].
These are examples of images of some of the very distant super novae taken with
Hubble space telescope. This is just to indicate how difficult
these observations are. Not only do these faint supernovae have
to be discovered, they have to be discovered early, their light curves
mapped, their red shifts obtained, all of the different corrections applied, and
only then they can be used in Hubble diagram.
All of this requires a lot of effort by the best people and the best equipment
that we have. Anyway, that was done in mid 90's, by 2
groups working independently. One was the supernova cosmology project,
at Lawrence Berkeley Lab, led by Saul Perlmutter, and the other one, was, High
redshift Supernova Team, led by Brian Schmidt, and Adam Riess.
The 2 groups, essentially produced the same result, at roughly the same time.
And more or less, independent. They both found out, that the high
redshift supernova Hubble diagram, requires introduction of the cosmological
constant or vacuum energy density in order to fit the data.
Whereas, there was a lot of circumstantial evidence for cosmological
constant prior to that. This was the first time that majority of
physicists really started paying attention and taking this possibility
seriously. Here we see Hubble diagrams with the main
trend line subtracted a particular model subtracted and then residuals fit for
variety fodder models. The model that fits the data best, is the
one that includes cosmological constant, at a level of approximately 70% of
critical density. This was a spectacular result, and the 3
leaders of the groups, deservedly got Nobel Prize, for that work.
Although, again, it's important to point out, that These are large, team efforts,
and not necessarily reducible to a handful of individuals.
This is how science today operates. Here is the Hubble diagram replotted now
in the original traditional sense of scale factor versus time.
And so, the points extend to the past, you can see that they select a particular
curve And that curve, corresponds to, cosmological constant, driven universe,
that expands forever. Many other groups, have undertaken this
work since then, and they all pretty much agree.
The results just get better, and better. Here is an example of one of the more
modern High redshift supernova Hubble diagrams, and colors of points correspond
to those, from different groups. As you can see, there is an excellent
mutual agreement, and the signal to noise is really getting to be very good.
So this was the upshot, of the supernova Hubble diagram.
On the plot of Critical dense, I'm, on the plot of, density parameters, matter
on the x-axis and vacuum or cosmological constant on the y-axis.
The measurements define an error ellipse, which is tilted and the plot universe
corresponds to a diagonal straight line, as shown here.
They do intersect and intersect roughly at density of matter of .3 for critical
density and density vacuum about 0.7 for critical density.
So this is the 1st time it was possible to say that if the universe was flat,
then there must be a cosmological constant.
But note that these error ellipses are pretty large and they do not really nail
down the exact cosmological model. The zero value of cosmological constant
is sufficiently far from the best fit that it can be excluded.
But exactly what mixture of vacuum density and matter densities at play had
to be determined using some other method. An interesting twist to this is to use
cosmic gamma ray bursts as alternative standard candles, these are even more
spectacular explosions. But it turns out, that their gamma
variable luminosity can be also standardized to a nearly constant value
using a variety of other parameters. At any rate, it's an independent test on
what supernova results are and here are the error contours from gamma ray bursts
alone. There are bigger than those from
supernovae because gamery versus not quite a good standard candle but they
produce more or less the same result. Now few words about the other classical
cosmological test, the angular diameter test.
Here we compare relative angular diameter distances to source as the different
[UNKNOWN], and in the past things like isophotal sizes of giant elliptical
galaxies, mean seperation of galaxies in clusters, or seperation of radio lobes in
radio sources, we're all suggested that as standard rulers.
And non of them really is for the same reasons as before evolution.
Here is some of the earlier results trying to do that and interestingly
enough all of these have also in consistent in what we now know is the
correct cosmology. But People simply could not be sure,
because of the evolutionary effect that was not well understood at the time.
This test got revived through the cosmic microwave background.
And it's probably now the most important of all of our tools.
So next time we will talk about cosmology Cosmic microwave background fluctuations.
What really led us into the era of precision cosmology.