Thank you to Anyesk for supporting PBS. The biggest news in cosmology in recent
years is that the mysterious universe
accelerating entity that we call dark
energy may be fading away. The evidence
for this is now strong enough that
enormous effort is going into confirming
the result. So, what's it going to take
and when are we going to know?
the description. Okay, on to the episode. In the '90s, we thought
cosmology was solved. The universe is
expanding and slowing down under the
influence of gravity. In an effort to
nail down this result, astronomers
tracked the expansion history of the
universe by watching the flashes of
distant supernova. They found that
expansion wasn't slowing down at all. It
was speeding up. We don't know what
mysterious influence causes this
acceleration, but we've named it dark
energy. The most mainstream
interpretation is that the vacuum of
space has a native energy to it. Quantum
mechanics predicts this to be the case,
even if it can't predict the quantity.
This idea also gives us the simplest
mathematical description of dark energy,
the addition of a single number, the
cosmological constant lambda, to the
Einstein equations. And we saw in
another video that this result is a
constant energy density and a resulting
exponential expansion. This simplest
version of dark energy is part of the
lambda CDM cosmological model. And
historically, it's done a pretty good
job of describing the expansion history
and the growth of structure through
cosmic time. With lambda CDM, we thought
that we were back to cosmology being
mostly solved. Time to tie up the loose
ends once again by refining the
measurement of the expansion history to
verify the dark energy is indeed
constant. Many thought that the dark
energy science instrument would do just
that. Confirm lambda
CDM. It didn't. Desi seems to be showing
the dark energy isn't constant at all.
Rather, it may be decreasing in
strength, which puts in doubt the whole
vacuum energy thing.
We first reported this result last year
after Desi's first data release when the
hint of a changing dark energy was just
that, a hint. Desi's second data release
has now promoted this tenuous result to
a confidence level just shy of a formal
detection. That was a few months ago
now. And so this is not a news piece
about the new result. Instead, this
episode is really about what comes next.
What measurements can and will we make
next to confirm that dark energy is
changing to better measure how it's
changing and what this can tell us about
what dark energy really is. That said, a
quick overview of DESIE is in order. The
dark energy science instrument is an
instrument. It's the survey conducted by
that instrument and it's the nearly 500
person collaboration behind it all. The
instrument is on the male telescope at
Kit Peak in Arizona. It robotically
places 5,000 optical fibers to
simultaneously capture the light of as
many galaxies. That light has traveled
to us for billions of years in some
cases through an expanding universe. And
the signature of that expansion is
imprinted in the stretching or red shift
of each galaxy's light. By splitting the
light of each galaxy with a
spectrograph, the Desi team can measure
that red shift, which encodes both the
distance to each galaxy and the
expansion history during that light's
journey. These two properties can't be
disentangled without more information.
And so, Desi does something else
incredibly cool. Remember, we want to
measure the expansion history. Perhaps
the simplest way to think about it is
that we want to measure how big the
universe was at different times in the
past. Light takes time to reach us. So
light from distant galaxies carries a
snapshot from an earlier time. Next, we
just need some sort of structure that
has expanded with the universe so we can
measure the size of the universe at
different times. In the case of Desi,
that something is the baron acoustic
oscillations, BAO, which we discussed in
previous episodes. These are gigantic
circular patterns in the distribution of
galaxies on the sky. They are the
remnants of sound waves in the hot dense
plasma that filled the early universe
before the first galaxies formed. These
density waves froze at a certain size
when the first atoms formed, leaving a
ripple pattern in the gas and dark
matter that filled the universe. The
galaxies that would later form from that
matter still reflect that patterning.
But the ripples have grown with the
universe. They now give us a powerful
standard ruler to track that
expansion. So simplistically, we have
red shift that encodes the subsequent
expansion history from a given time in
the past and then we have a measure of
the scale of the universe at those
times. This doesn't quite give us an
unambiguous expansion history by itself,
but the combination does allow desi to
test the consistency of different models
of that history, different cosmological
models. In the case of Desi, the
cosmological models tested include
lambda CDM with its constant dark energy
as well as a particular model for
changing dark energy. In that model,
dark energy changes over time according
to an equation given in the form of a
varying equation of state parameter.
Something we talked about in the
previous video. By itself, Desi finds
that the model with changing dark energy
has only a marginal improvement over
lambda CDM. But things get much more
interesting when the desi data is
combined with other studies. The first
is the constraints from the cosmic
microwave background radiation which
provides the starting point the initial
conditions from which cosmic expansion
and structure formation measured by desi
will evolve. The other is an independent
measure of distance. Desi itself only
has the galaxy red shift as a distance
indicator which makes it difficult to
pin down the absolute scale of these BAO
rings. So they add an independent
measure, the same type that was first
used to discover dark energy. Type 1A
supernova. These exploding white dwarf
stars are called standard candles
because they explode with predictable
brightnesses. That means we can figure
out how far away they are based on how
faint they appear due to that distance.
One way to think about it is that the
supernova give distances from us to the
galaxies while the barrier acoustic
oscillations give distances between
those galaxies. Standard candles plus
standard rulers. Combined with the
initial conditions from the CMBB, these
measurements give a much tighter
constraint on their cosmological
parameters. Desi claims that the
combination favors a cosmology in which
dark energy is actually weakening and
that this differs from the cosmological
constant model by up to 4.2 sigma for
the supernova in Desi's own survey, but
a bit less than 4.2 for other supernova
surveys. This doesn't quite reach the
five sigma level required to claim a
discovery, but it's tantalizingly close.
So, did Desi probably discover that dark
energy isn't constant? Not really.
Besides not quite hitting five sigma,
they also test this one version of
varying dark energy, which you could
argue is a little ad hoc. When you're
fitting a curve to a bunch of points,
adding extra degrees of freedom always
improves the apparent fit. That could be
what's happening here. Still, it's
intriguing enough to warrant deeper
exploration. What we really want is a
precision measurement of the way
expansion and dark energy have changed
over cosmic time because that would
allow us to a be sure that the change is
really happening and b distinguish
between different models of dark energy
which in turn could tell us what dark
energy really is. Now we've talked about
those options before so I won't go into
it now. I want to focus on what's needed
and what's being done to improve this
measurement. How do we improve the desi
result? Well, the answer is a little
prosaic, I'm afraid. We just need to do
everything better. Let me remind you of
the three things we talked about. Now,
this is an oversimplification, but it
serves to organize the ideas. We need to
one measure the starting state of the
universe, two to measure the size of the
universe at different times, and three
to measure distances to those different
times. We'll dispense with number one
quickly. Measuring the starting state
means measuring the initial density,
distributions, and various energetic
components of the universe from the
cosmic microwave background. There are
plans to do this better than the plank
satellite already has, but honestly, our
current CMBB map is the most precise
part of this whole calculation. So, it's
not the limiting factor. So, straight on
to number two. How can we improve our
measurement of the size of the universe?
Well, Barryon acoustic oscillations
really are the gold standard and so just
doing desi better is probably the way to
go. That means we need bigger, better
galaxy red shift surveys which beat down
the uncertainty in various ways
including the sheer number of galaxies.
We also want to push the measurement
back further in time, which we can do
with more sensitive instruments on
bigger telescopes or just staring at the
sky for longer to collect photons from
fainter galaxies. And three, we also
want to improve our measurement of
distances to those galaxies. The most
obvious thing is to improve our
supernova measurements with more type
1As, but also by refining our
calibration of these things as standard
candles. This is one of the most
contentious points in the whole effort
because the supernova distances
piggyback off a chain of distance
measurements and so errors can have
crept in unnoticed in various places.
Major efforts are underway to weed out
potential issues.
It's also important to find independent
ways to get those distances. One of the
most exciting and a personal favorite of
mine is to use gravitational lensing.
Gravity bends space and so bends the
path of light. Very strong gravitational
fields like those of entire galaxies
bend space so much that the light from a
distant source can travel to us via
multiple separate paths. For example,
distant quazars feeding super massive
black holes sometimes appear duplicated
or even quadrupled around a galaxy that
happens to lie directly between us and
it. Now, when that quazar fluctuates in
brightness, all of these lensed images
also fluctuate because they're all the
same quazar. But the pattern of
fluctuations has an offset in time that
reflects the difference in the lengths
of these paths. Measuring this time
delay actually gives us a measure of the
cosmic distances that's completely
independent of the supernova
measurements. Astronomers have used a
handful of lensed quazars to measure the
expansion rate of the whole universe.
But by doing this for many more such
objects, we can also get the expansion
history. Now, I was being a little
simplistic when I separated the ideas of
measuring the size of the universe at
different times to measuring the
distance to those times. Really, any
given cosmological measurement does some
of both. For example, we track the size
of the universe just by looking at how
far apart galaxies are in general, not
just the size of the BAO rings. So, this
size information is also there, for
example, in the supernova cataloges,
even without the BAO analysis. And the
expansion history is coded in the red
shifts. So we can calculate expansion
rates even from just say the lensing
time
delays. But really combining these data
sets is the key. What we're trying to do
is fit a cosmological model with many
interacting or covariant parameters. So
we need measurements that will tease
apart and constrain all parameters. And
on that note, there is one parameter
that I haven't mentioned, and that's the
clustering of galaxies. Over cosmic
time, galaxies are thrown apart by
cosmic expansion, but they also fall
together due to their mutual
gravitational attraction. The amount of
this clustering tells you stuff about
the amount of dark matter in the
universe and the size of the initial
density fluctuations, both important
parameters in your cosmological model.
So measuring this clustering is a big
help in constraining that model. Galaxy
red shift surveys like desi can do this
directly to some extent based on the
clustering of the galaxies that they can
see. But there are also more cunning
approaches that can take account of all
the mass seen or
unseen for example through weak
gravitational lensing. The lensing I
already talked about where you see
multiple images of one object is called
strong lensing. In weak lensing, the
gravitational field of galaxies and
clusters slightly warp the shapes of
background objects without actually
producing multiple images. In the case
of a weakly lensed galaxy, it'll be
slightly stretched in one direction. You
can't identify the effect with a single
galaxy because you don't know what shape
that galaxy was originally. But imagine
many galaxies in a particular direction
that normally have random orientations
relative to each other. As their light
travels to us, it passes around, let's
say, a particularly dense patch of the
universe. The orientations of all those
galaxies will be skewed at right angles
to that patch and become correlated with
each other. By looking at the
correlations of galaxy shapes across the
entire sky, you can get a crude density
map of the universe, which in turn
allows us to constrain galaxy
clustering. The desi result that made
all this news is from their second data
release, 3 years into the 5-year survey,
and with 30 out of the planned 50
million galaxies and quazars. Actually,
if the deviation from constant dark
energy is real, it seems likely that
this extra signal will get them over the
five sigma level. But Desi is not the
only game in town, nor the only thing
needed to reduce that uncertainty. We
should not confuse Desi with DE, for
example. That's the dark energy survey.
Des observes a factor of 10 more
galaxies than Desi. But where Desi takes
spectra of known galaxies, Des takes
images at different wavelengths and
actually discovers galaxies. That makes
DE an imaging or phototric survey where
DESIE is a spectroscopic survey. DE
measures phototric red shifts which are
less accurate than DESI's spectroscopic
red shifts but take much less time to
observe. DE also doesn't see as far as
Desi. so isn't sensitive to the
expansion history. Instead, DES is good
at measuring the latetime expansion rate
as well as the growth of structure via
clustering and weak lensing
measurements. It's also important for
nailing down them pesky supernova.
Really
desime constraints just as the CMBB
gives the initial conditions. But
perhaps the greatest improvement on
these results will be from very new and
upcoming surveys. We have the European
Space Ay's Uklid satellite which is
studying over a billion galaxies with
both imaging and spectroscopy and his
most important cosmological contribution
is by measuring galaxy clustering and
weak lensing. This should allow us to
beat down the uncertainty in the
equation of state parameters to the 1%
level which moves us toward a precision
measurement of dark energy. Uklid is in
the first of its six-year mission and
the first cosmological results are
expected next year. And the last and
perhaps for me most exciting endeavor is
the legacy survey of space and time LSST
on the Reuben Observatory. This is a
phototric survey that does something
very new. It'll map much of the southern
sky many many times for 10 years.
Besides complimenting or improving on
desi deaths and uklid lssts will add the
dimension of time to the analysis.
Perhaps the most important benefits to
cosmology are the measuring of many
hundreds of thousands of type 1a
supernova providing an incredible late
time anchor for the other surveys. LSST
will also enable measurement of those
time delays in the gravitational lens
quazars which if you remember give us an
expansion history independent of
supernova. But LSST will only do this
after discovering something like 20
times more lensed quazars than we
currently know of. So this method should
really come into its own with LSST. We
are entering the era of precision
cosmology. It's really a golden era for
the field. And in this next decade, you
can expect the cosmic expansion history
and the general cosmological model to
become known to a precision that seemed
impossible not long ago. With that
precision comes the ability to ask what
kind of physics can produce such a
universe. What kind of dark energy, dark
matter, starting conditions, and what
does that mean for how the universe will
end? We will know it all just as soon as
we've finished surveying all of
spaceime. Thank you to Anyesk for
link in the description.
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