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The currently accepted cosmological description of our universe is called
the Lambda CDM Model and is built on the idea about the behavior of Dark Energy and Dark Matter.
It’s accepted because it does a great job of explaining our observations of the universe.
That is, perhaps, until now.
Subtle clues are emerging that the accepted model for the nature of dark energy and dark
matter may not be all that.
We saw the first such clue recently in our recent episode on the Crisis in Cosmology.
Today we’re doing a Space Time Journal Club to reveal another clue.
We’re looking at a new paper in Nature Astronomy, “Cosmological constraints from the Hubble
diagram of quasars at high redshifts” by Risaliti and Lusso.
It hints that the cosmological constant may not be so constant after all.
In fact it may be increasing.
If this is true, then our prediction for the future of our universe looks VERY different,
and may involve the entire universe tearing itself to shreds at the subatomic level in
the Big Rip.
Crazy right?
But hopefully before that happens, let’s try to understand it.
First we’ll review the current, accepted model, then the problems with it, and finally
this new study which tries to resolve those problems but may have made things way more
mysterious.
Back in the late 90s two teams of astronomers accidentally discovered dark energy.
They were measuring the distances to supernovae to track the changing size of the expanding
universe.
Instead of slowing down, as was expected, that expansion turned out to be accelerating.
It was revolutionary and radical at the time, but a few Nobel prizes later and dark energy
is now textbook cosmology.
Those textbooks tell us that we can this accelerating expansion is what you expect if empty space
has a constant energy density – so that more space means more dark energy.
Mathematically we represent a constant vacuum energy with Einstein’s cosmological constant
– or Lambda.
Those same textbooks talk about dark matter – an invisible stuff whose gravitational
influence overwhelms all types of visible matter combined.
We see its effect in the rotation and movement of galaxies and in the bending of light due
its space-warping gravity.
Decades of study and calculation suggest that dark matter is a particle of some unknown
type, cold, diffuse, and immune to electromagnetic interactions of any type.
In the textbooks, this type of cold dark matter sits alongside the cosmological constant as
our best description of how the universe behaves on the largest scales.
Constant dark energy, cold dark matter – or the Lambda-CDM model.
We also call it the Concordance model - a term that just means the current best-accepted
picture.
OK, so now to the crisis, which we look at in detail here.
Our observations of the cosmic microwave background reveal the starting conditions of the universe
– the balance of dark energy, dark matter, and everything else at the earliest of times
when the CMB was released long before the first stars formed.
So we apply the Concordance model to these starting conditions to calculate how the universe
should have evolved from those early times, and how fast it should be expanding today.
This prediction doesn’t seem to agree with our observations.
Let’s talk about those observations.
The expansion history of the universe is typically measured using the same type of supernova
observations that first discovered dark energy.
Type-1a supernovae– which result from exploding white dwarf stars.
These explode in very predictable ways, releasing a predictable amount of energy.
By comparing that expected energy output with the amount that actually reaches our telescope,
we can figure out pretty precisely how far away the supernova was when it exploded.
Type 1a supernovae are what we call standard candles – objects of known brightness that
can be used to find distance.
If we have lots of supernovae at different distances when we have a set of rulers spanning
both time and space.
The most distant supernova we’ve seen is so far away that its light has been traveling
to us for around 75% of the age of the universe.
Our rulers span much of cosmic time, and together they give an expansion history.
Taken on its own that history just what we expect in a Lambda-CDM cosmology.
The cosmic microwave background is also consistent with Lambda-CDM – but the exact numbers
are different.
In particular, the universe appears to be expanding faster than expected given what
we see in the cosmic microwave background.
Something is wrong.
It may be an issue with how we determine the starting conditions of the universe, or it
may be our measurements of supernovae.
Or it may be that Lambda-CDM is wrong.
Perhaps the cosmological constant is not so constant, or dark matter is not so cold after all.
But before we throw away the textbooks, perhaps we should think about where the measurements
might have gone wrong.
Perhaps surprisingly, our measurement of the state of the early universe via the CMB may
be more reliable than our measurement of its subsequent expansion history via supernovae.
Part of the reason for that is that supernovae aren’t bright enough to see through ALL
of cosmic history.
We miss the first 25%, and we’ve found too few supernovae over the first 50% of cosmic time.
That early period is extremely important for understanding the true behavior of dark energy.
This motivated Risaliti and Lusso to seek a completely new way to measure that expansion history.
In order to properly measure the full expansion history of the universe -
we want a new, brighter standard candle.
Enter the quasar.
These things are by far the most awesome of all astrophysical phenomena, as we conclusively
demonstrated in this episode.
Quasars are a black hole feeding frenzy.
When matter falls too close to a black hole, it forms a superheated vortex pouring into
the black hole – an accretion disk.
If it’s a supermassive black hole doing the feeding, its accretion disk shines so
bright that it can be seen to the ends of the universe.
We call such objects quasars.
Quasars are, at first glance, pretty crappy standard candles.
They can have a huge range of energy output, depending on the mass of the black hole and
how much fuel it’s getting.
It’s very hard to distinguish between a brighter quasar that’s further away or a
fainter quasar that’s closer to us.
In general quasars are a hot mess.
Literally.
It’s not just the accretion disk – the energy produced in that disk powers all sorts
of crazy energetic activity.
There are vast winds that spray into the galaxy, giant jets that can punch out into intergalactic
space, and also, hovering above the accretion disk we have a hot atmosphere that radiates
X-rays.
This last one – the X-ray corona – may help us make sense of everything.
The new study uses the brightness X-ray corona to figure out the true energy output of quasars.
It works like this: when photons of ultraviolet light radiate from the accretion disk, they
bump into extremely energetic electrons in this region above the disk.
In that encounter they can get boosted to even higher energies – right up to X-rays
– in a process called Compton up-scattering.
All quasars shine in X-rays due to this process, and as you might expect, the brighter a quasar
is in ultraviolet light the more in X-rays it can produce.
That doesn’t sound particularly helpful just yet.
If UV and X-ray light track each other perfectly, there would be no way to differentiate the
effects of distance and intrinsic energy output.
Both effects would cause the UV and X-ray light to brighten or dim in the same way.
Except here’s the weird thing – UV and X-ray DON’T track perfectly.
There’s a diminishing return in how much X-ray you can squeeze out of the X-ray corona.
As UV brightness goes up, X-ray brightness does increase, but not one-to-one.
That sounds like an obscure point, but this is going to give us our standard candle.
See, the ratio between the amounts of X-ray versus ultraviolet light depends on true ultraviolet
energy output of the quasar.
That means you can just measure that ratio – the relative brightness in X-ray versus
UV – and you know the true UV brightness.
When compared to observed UV brightness that gives you distance.
Now there is a bit of variation in the relationship between UV and X-ray brightness – there’s
random scatter.
But if you make the measurement with enough quasars then this scatter can be averaged away.
Fortunately there are a LOT of quasars out there – far more than known supernovae.
Risaliti and Lusso cobbled together around 1,600 quasars with both ultraviolent and X-ray observations.
Most where from giant surveys like the Sloan Digital Sky Survey and the XMM-Newton X-ray catalog.
They also added extra X-ray observations with the XMM-Newton satellite.
The final sample spans nearly the entire history of the universe.
The most distant existed when the universe was less than 10% of its current age.
This graph is from the paper.
It’s what we call a Hubble diagram.
It basically shows the distance of the quasar that we get from the UV-to-X-ray ratio,
versus the redshift, which is how much the quasar’s light got stretched as it traveled the expanding universe.
Putting these two together measures the expansion history.
The yellow and blue dots are our quasars – 1600 of them.
The red dots show the average distances in redshift bins.
And that dashed line – that reflects the expansion history expected in a universe with
constant dark energy – a Lambda-CDM, concordance universe.
The red points are consistently below the dashed line for large distances and redshifts.
That means the light from these quasars appears more stretched out – more redshifted on average –
than it should be given their distance, AND given textbook Lambda-CDM cosmology.
The solution?
Well, that black line is a model of the expansion history of the universe in which dark energy
is NOT constant, but instead is getting stronger as the universe ages.
It fits the data pretty well.
Very roughly speaking – if the expansion of the universe is accelerating even more
than we thought, that could explain the extra stretching of the light from these distant quasars.
So what does this mean?
Are concordance and the cosmological constant dead?
Is dark energy getting stronger?
Is the big rip about to happen?
Let’s start with the latter, just in case.
At the moment, dark energy is only strong enough to accelerate the expansion of space
on the largest scales.
It’s not strong enough to have any effect on the space inside a galaxy, so the Milky
Way, and certainly the solar system are safe.
But if dark energy is getting stronger, then eventually it could cause the universe to
expand inside galaxies, inside planetary systems, and eventually even inside atoms.
This scenario is known as the Big Rip, and we will probably do a whole episode on it
at some point.
It’s a potential end of the universe in which space-time rips itself to shreds at
subatomic scales due to the increasing strength of dark energy.
Before you dust off your “end-is-nigh” sandwich board a couple of things:
1) any potential Big Rip is tens of billions of years away.
2) if dark energy has changed in the past that doesn’t mean that it’s steadily increasing.
In fact there are alternative ideas that try to resolve the conflict between the CMB, supernova,
and now the quasar results.
For example, there’s the idea that dark energy that started out much stronger dropped
off rapidly, or even that it oscillates over time. All very exciting.
But the most likely explanation is still that concordance, Lambda-CDM is right, and there’s an issue
with the measurements or calculations.
Perhaps a systematic problem with the way supernova and/or quasar distances are determined,
or even an issue with the cosmic microwave background calculations.
Also there are still relatively few very distant quasars with good X-ray measurements, so maybe
the random variations in the UV-to-X-ray ratio by chance to gave a false result.
These issues will be resolved with more X-ray observations and more testing of the new technique.
What we have here is a tantalizing clue that our accepted understanding the factors that
drive the expansion of the universe may be off.
Perhaps this is the clue we need to finally better understand this stuff we call dark energy.
And even if the Concordance model ends up reigning supreme– even if dark energy proves
to be constant after all – which I suspect it will – we’ll have a new, independent
measure of its existence and behavior.
One way or another I guess it’d be nice to know whether dark energy will one day rip
to shreds the subatomic fabric of space time.
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All you have to do is click on the link the description below, to start today
Last week we had a conversation with Richard Branson about his recent successful tests
of SpaceShipTwo, which recently carried the first private passenger into space on a private craft.
Richard also shared his dreams for the future of human space travel.
You guys shared your own thoughts too.
A few of you dispute the use of the term spaceship for a craft that doesn't reach orbit.
Indeed, Virgin Galactic's Spaceship-1 and Spaceship-2 crossed the boundary into space
- past the higher Karman line in the case of spaceshipone.
But neither entered orbit.
They followed suborbital trajectories, which left them in space for only minutes.
Now in my book if it shipped you into space it's a spaceship, but these terms aren't well
defined yet, so you can insist on your definition too.
That said, Virgin Orbit - a spin-off of Virgin Galactic has air-launch rocket that is expected
to put satellites in orbit this year.
IT's called launcherone, and it'll launch from WhiteKnightTwo - the same plane that
carries spaceshiptwo.
Paul C mentioned a couple of other promising spacecraft that aren't getting as much attention
as they should.
We didn't have time to fit more in this episde, but there's a ton of cool stuff happening.
Paul mentions Stratolaunch - an air-launch plane - MUCH bigger than the SpaceShipTwo
launcher and designed for orbital rocket launches as well as space planes.
THis is another project of Burt Rutan - designer of the Virgin Galactic craft.
Dustin Edwards hopes I got to kitsurf on Richard Branson's private island.
Actually, I did! thanks Dustin.
Well, I kited - still working on the surfing part.
Many of you asked me to elaborate on how I ended up on that island in the first place.
Happy to oblige.
I flew to San Juan and then Tortola, followed by a boat to Necker island.
Pretty mundane really.
Getting off is another story. Branson himself actually kitesurfs from the island.
I mean, when the spaceplane is in the shop.
A surprising number of you asked whether I was hunted like an animal across the island,
which is apparently what happens on billionaire private islands.
No, nothing like that happens on Necker Island.
It's more the tennis and watersports style of billionaire private island, less the hunt-human-chattal-for-sport type.
What happens on other billionaire private islands ... who can say?
From what I hear about Bill Gate's island ... actually, I've never heard ANYTHING about
that island.
Suspicious.