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.

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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.