Thanks to the Great Courses Plus for Supporting
PBS Digital Studios. 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. Thanks to The Great Courses Plus for supporting
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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.
This was a really good episode. It's so cool that cosmology still has so many uncertainty's. Makes me want to become a cosmologist!
Old entry video! Nice! And I liked that they explained what the diagram means. I could understand the whole episode :D
There is something I've been wondering and it might be a little dumb.
How do we know dark energy is an intrinsic energy (energy per volume of space) of space time and that the accelerating growth of the universe is not caused by some kind of transcendent force that's applied from outside our universe ? In which case for all we know it could be somewhat erratic.
I guess it must be because if this force is transcendental and outside of our grasp then it wouldn't be able to affect us (maximum speed of causality, ie light speed) ?
I just started watching the spacetime videos - I'm a few years behind so I apologize if this is answered at some point (I'm currently on "Why the Universe Needs Dark Energy" from 2016-04-20) . But I have a question.
Let's say that dark energy can't interact with normal matter at all and can only interact with spacetime itself. There's nothing that can detect or interact with it at all... AKA, it's outside of causality.
Could it then break causality? Could it ignore the causality speed limit (aka 'speed of light')?