Thank You To Radiacode for supporting PBS.
The universe is expanding and that expansion is accelerating under the power of dark energy and
eventually all matter and energy will be dispersed over such unthinkable distances
that nothing can stop space from blowing up infinitely. Unless of course cosmologists
blundered and dark energy doesn't even exist. Then it's back to the drawing board.
Hey everyone, quick update before we start! We've observed that the
YouTube algorithm isn’t promoting our episodes within the community as much,
so we’re conducting an experiment. Over the next few episodes, we’ll test whether
increasing likes and comments impacts sharing. If you'd like to be a data point in this experiment,
hit like and/or leave a comment. And if you’re new and enjoying this,
subscribe and please press the bell. We’ll analyze the results and report back after a few episodes!
A paper came out in December claiming that dark energy—widely held to be
the dominant form of energy in the universe,
and one the the central components of our modern cosmological model—simply doesn’t exist.
The paper, by Antonia Seifert and team at the University of Canterbury in New Zealand,
claims that the apparent accelerating expansion comes from an oversimplification of our model of
the expanding universe. The researchers state that the current Lambda-CDM model doesn’t properly take
into account the fact that time flows differently in different parts of the universe. In particular,
time flows slower in regions of strong gravity—so in the dense clusters and filaments of the cosmic
web. And it flows faster in the vast, empty regions in between. This results in a landscape
of shifting time flow—in which some parts of the universe may be billions of years older
than others. So, instead of a landscape, more of a timescape. The timescape model was first proposed
by David Wiltshire in 2007, and in this picture, faster-ticking parts of the universe will have
expanded more than slower-time parts, and this effect can exactly mimic the apparent accelerating
expansion, but without any acceleration at all at least according to this team.
Now, people have been saying that dark energy doesn’t exist since its discovery, with varying
levels of quackery. This particular proposal got more attention than usual, in part because
it’s definitely not quackery. The proposal is scientifically well-founded and the researchers
are serious people. There’s also a certain elegance to it that I hope I can convey to you.
None of this means the idea is right, and I’ll also try to convey why it’s probably not true.
First the super-quick review on dark energy. The universe is expanding,
and we’ve measured the history of that expansion. One of the most important ways we
do that is by measuring the distances to Type 1-a supernovae. These exploding white dwarf stars pop
with predictable brightnesses. Light from these explosions travels to us over great distances,
and two things happen on that journey. One is that the light is dimmed as photons spread out
through the universe. This tells us how far and for how much time the light traveled. Two is that
those same photons are stretched—redshifted—by the expansion of the universe as they travel.
This tells us how much expansion happened during their travel time. Put these together for many
supernovae back through cosmic time and we get an expansion history for the universe.
The results of the first big supernova surveys came in in the late 90s, and scientists had
expected to find that the expansion rate was slowing down due to gravity. It had to be because
Einstein’s general theory of relativity only allowed an inward-pulling gravitational force.
To everyone’s great surprise, these studies found that the expansion was accelerating.
At first glance this made no sense under the then-current version of general relativity.
However, adding an additional piece to GR—the cosmological constant—provided exactly the
acceleration needed to explain the observations. We don’t really know what causes this accelerating
expansion—top contender is some sort of energy of the vacuum itself. But whatever it is, we
call it dark energy and its discovery scored our supernova collectors the Nobel prize in physics.
The existence of dark energy in the form of a cosmological constant is now part of our dominant
mathematical description of how the universe works on the largest scales— the Lambda-CDM model.
Lambda-CDM does such a good job of explaining a whole range of observations that it’s hard
to avoid the conclusion that something like dark energy must exist. On the other hand,
we’ve never really measured cosmic acceleration directly. We’ve just
found that models with dark energy fit the data better than models without it. But what
if there’s some other founding assumption behind our model of cosmic expansion that
is wrong? That’s basically what the timescape model is saying.
To understand the different factors driving the expansion of the universe, we need to solve the
equations of general relativity for the entire universe. That’s only possible with some pretty
major simplifications. The main one is that we assume that the matter in the universe is spread
out smoothly everywhere—it’s both homogeneous and isotropic—no large-scale lumps or directionality.
That lets us describe the effect of matter with a single number—the matter density—rather
than having to solve for every chunk of matter separately. The result is the Friedmann equations,
which we’ve picked apart before, and they describe how the size of the universe changes over time,
driven by smoothly distributed matter and constant dark energy. Add the appropriate
amounts of both and you can describe our universe—and that’s the Lambda-CDM model.
Although the approximation of smoothness seems to be very good on large scales,
we know that the universe is not smooth on smaller scales. At distances smaller than
a billion light years or so, this smooth matter has fragmented into enormous galaxy clusters and
the filaments of matter connecting them. And bound by this cosmic web are giant empty patches—cosmic
voids. These structures do have an effect on the photons passing through them. For example,
photons gain some energy falling into deep gravitational fields, resulting in blue-shift,
and lose a little energy climbing out of them, resulting in redshift. So a supernova
that explodes somewhere deep in the Laniakea supercluster might gain some redshift from
traveling to us through the expanding space, and more redshift from the fact that it has to fight
against the supercluster’s gravitational field to reach the Milky Way, out on Laniakea’s periphery.
Local gravitational fields also affect the speeds of nearby matter, and so while that
supernova is mostly moving away due to cosmic expansion, it might have some extra velocity
either towards or away due to its home galaxy moving within Laniakea’s gravitational field.
We have to take care to account for this stuff,
particularly for supernovae in the relatively nearby universe. In fact,
we talk about the possible effect of local structure in a previous video.
But if the supernova happened more than a few hundred million light years away then its photons
will pass through multiple voids and superclusters on their way to us. All that blue-shifting and
redshifting will cancel out. If that’s the case, then the Friedmann equations and Lambda-CDM
should do a perfectly good job at describing the expanding universe experienced for those photons.
At least, that’s what most people think. The timescape model says otherwise. Like I said
at the start of this video, time flows slower in regions of high gravity and faster in low
gravity—this gravitational time dilation is a known and well tested feature of general
relativity. But the timescape model proposed that differences in time flow should have another
effect—it should change the rate of expansion of the universe from one place to another. It’s
straightforward enough. The more time that passes the bigger the universe gets, so if more time has
passed in a cosmic void compared to a region of higher density, that void will have expanded more.
This differential expansion has an effect on photons traveling through these regions. For
example, photons are stretched out or redshifted more as they travel through the faster-expanding
voids compared to dense regions. As matter becomes more and more clumped together over time,
the fraction of the universe with voids grows. This means that photons traveling through the
late universe spend more time in voids and so pick up more of this extra redshift.
In the Lambda-CDM model, it’s the accelerating expansion of the universe that gives photons
their extra redshift in later times. But in the timescape model, it’s the
increasing void fraction, and, supposedly, no global acceleration is needed in timescape.
The timescape model has been around since 2007 and so
the reason for the recent buzz is that the team just published their analysis of the results of
the biggest current supernova survey—Pantheon+. In general they find that timescape gives a better
fit to the supernova data than Lambda-CDM. You ca n see it on this plot, which shows the Bayes
factor—basically the ratio of how good timescape is compared to Lambda-CDM. If we’re in the blue
region then timescape is preferred. The x-axis is the redshift cut. Points on the left use all
the supernovae, including the nearby ones. As we go to right we cut out supernovae starting
from the closest to the more distant, so that over here we’re only including the most distant ones.
This is a nice way to show the result because the nearby supernovae should be more affected
by inhomogeneities—more influenced by the local lumpiness of the universe. So it makes some sense
that Lambda-CDM doesn’t do as well there, with its assumption of perfect smoothness. Further away,
that lumpiness averages out and timescape and Lambda-CDM provide similar results,
although according to this, timescape still does a bit better at all distances.
So, is this right? Has dark energy been disproved? Well, if the only evidence for
dark energy were these supernova results, then this might be bad news for dark energy
and the lambda-CDM model. That’s because, in some sense, the timescape model is simpler
than Lambda-CDM. It only uses raw general relativity, just as Einstein conceived it,
with no need for the addition of a cosmological constant. It uses the base theory plus the effect
of structures that we know exist in the universe. Occam’s razor tells us that if two hypotheses
yield identical results you should favor the simpler hypothesis. By that logic timescape
seems to win. The amount of time that lambda-CDM has been around should not,
by itself, make us favor the older theory. After all, Ptolemy’s Earth-centered model of the solar
system was around for a millennium and a half before Copernicus came along with his simpler
Sun-centered model. But the test of time is only relevant if it actually involves additional tests.
And indeed there are additional tests for Lambda-CDM. This model provides a
good description of more than just the supernova results. For example,
baryon acoustic oscillations—reverberating sound waves from very early times—are now frozen as
faint rings in the distribution of galaxies on the sky. The size of these rings gives an independent
way to measure the expansion rate that is not affected by how cosmic voids redshift photons.
And the BAO observations also strongly indicate accelerating expansion and support Lambda-CDM.
There’s also the evolution of large scale structure, which nicely match the simulations
we run under Lambda-CDM. And there’s the fact that the universe is geometrically very flat,
and for that to be the case we need 70% of the energy in the universe to be dark
energy. It may be that the timescape model can explain these too, but so far it hasn’t.
Another major issue with the timescape model is that it requires quite large differences
in timeflow between the cosmic voids and the dense regions of the universe—large enough that
billions of years more would have passed in the voids compared to the superclusters. Such large
differences are not the consensus. For example, Ethan Seigel points out that the relative flow
of time between these regions should result in an age differential of only 100s to 1000s of years,
which isn’t anywhere near enough to produce the effect that timescape claims. I should also
point out that the researchers themselves admit that more work needs to be done. For example,
supernova distances are not at all easy to figure out, and require many steps of
calibration. The timescape team acknowledges that some of this requires further work.
So, yeah, dark energy didn’t go away. Lambda-CDM still gives us the most consistent description
of the large-scale universe, across multiple independent observations.
At the same time, Lambda-CDM has its challenges—there’s the now-famous Hubble
tension—a mismatch between the expansion rate measured from supernovae versus the rate inferred
under Lambda-CDM from observations of the cosmic microwave background. And then there’s the recent
Dark Energy Survey Instrument result that the cosmological constant may be actually changing
over time. It’s not clear that timescape can sort out these issues, but any chink in the
armour of Lambda-CDM is a good motivation for exploring other models. And maybe part of the
solution will for us to be more careful in how we model the temporal landscape of spacetime.
Thank you to radiacode for supporting PBS. If you’re looking for a new science based hobby,
Radiacode is a gadget that’s made for all natural science enthusiasts. Radiacode is a
portable radiation detector that collects data fast. Now you can not only quickly locate the
source of radiation but also identify which isotopes are present, such as Cesium-137,
Thorium-232, Radium-226, and many others. The multifunctional app, included with the device,
works on iOS or Android and makes it easier to explore and research by allowing you to
record radioactive GPS tracks on maps, keep a measurement log, analyze spectra, create charts,
and build spectrograms. You can learn more about RadiaCode by following the link in the description
and the pinned comment below. Use the promo code "PBS" to get an exclusive 10% discount.
Hey Everyone. Before we go we wanted to let you know, we’re celebrating the 50th anniversary of
the Space Shuttle patent, with some limited edition merch at the merch store. There’s
multicolored hoodies and t-shirts featuring the original patent, the first ever space
time notebook as well as a re-release of the mutli-functional space time pen in new colors.
You can check it out at pbsspacetime.com/shop. There’s a link in the description.