Astronomers working with the NASA’s Chandra X-Ray Observatory and ESA’s XMM-Newton have
developed a technique to watch quasars and track the expansion of the Universe over the
last 9 billion years.
What they found is that the mysterious dark energy that’s currently accelerating the
expansion of the Universe doesn’t appear to be a fixed amount.
It’s changing, and appears to be increasing over time.
If true, it’s a groundbreaking discovery in cosmology, and it could just fortel the
end of the Universe.
Time for a quick backgrounder.
Then we’ll get into the new observations.
Back in 1998, astronomers were trying to measure the expansion rate of the Universe to try
and predict what the future would hold.
They assumed that the mutual gravity of everything in the Universe would decelerate the expansion
into the far future.
Would it slow to a halt and then reverse, eventually leading to a Big Crunch?
Would its expansion continue to slow down until finally coming to a stop at an infinite
amount of time?
Or would it just keep on expanding, never coming to a halt, ever?
In order to get to the bottom of this question, astronomers observed a very specific kind
of supernova, known as a Type 1a.
These happen when a white dwarf star pulls material from a companion until it reaches
about 1.4 times the mass of the Sun.
Then it explodes as a supernova.
Because the supernova happens with a specific amount of mass, the explosion releases a specific
amount of energy.
Measure the brightness of a Type 1a supernova, and you know how far away it happened.
Astronomers charted out the brightness of these supernovae, trying to get a sense of
the size of the Universe at various times in its history, and it turned out the answer
was: none of the above.
The expansion of the Universe isn’t slowing down, in fact, it’s speeding up.
The expansion of the Universe is accelerating.
As a placeholder for this strange nature of the expansion of the Universe, astronomers
came up with the term dark energy.
They didn’t know what it was, and still don’t, but it appears to be some kind of
mysterious repulsive force that appears in every single part of space.
The more space there is, the more of this repulsive force that appears in the Universe.
Shortly after dark energy was discovered, astronomers came up with a new question.
Is this dark energy a constant force, or has it actually been changing over the history
of the Universe?
Imagine a cubic meter of space, and a tiny amount of pressure pushing out.
As the Universe expanded, the total amount of dark energy grew, but it was assumed that
the total force in each cubic meter remained the same.
But what if the force was actually changing?
What if it was increasing?
Billion of years ago, was the pressure half what it is today?
Will it be twice as strong in the future?
The answer to the question is actually pretty important, as it foretells the ultimate fate
of the Universe.
If this force is constant, then the Universe will continue to accelerate apart.
Galaxy superclusters will be carried away from each other, dropping off over the cosmic
horizon.
But for regions which are relatively close together, like individual galaxies, stars
within the galaxy, etc, gravity is the dominant force.
Dark energy can push away all it likes, but it can’t overcome the local force of gravity.
But if the force of dark energy is increasing, this means that regions dominated by gravity
will eventually be dominated by dark energy.
We might get to a point in the future where the Milky Way and Andromeda stop falling towards
each other and get pushed away.
Then, as dark energy continues to strengthen, the gravity of a galaxy can’t hold itself
together and individual stars are thrown out.
Then star systems are overcome by dark energy, tearing their planets out into the void.
Dark energy becomes stronger than the gravitational binding energy of stars and planets.
They get torn apart.
Molecules, atoms, subatomic particles.
Nothing can withstand it.
Keep increasing and even black holes are broken up into pieces and scattered into the ever
accelerating, expanding Universe.
You probably already know, this idea is known as “the Big Rip”.
Based on the most precise calculations astronomers have been able to do, it shouldn’t happen
any time in the next 80 billion years or so, many times longer than the Universe has already
been around.
But far sooner than the trillions of years that red dwarf stars might live for.
Or the Googol years it could take for the last supermassive black hole to finally evaporate
away.
Answer The Question Is this going to happen?
To the best of their ability, astronomers previously calculated “probably not”.
They were able to estimate that dark energy had remained constant over the history of
the Universe within 1-2%.
The most precise measurements of the accelerating expansion of the Universe came from those
supernova surveys I mentioned earlier.
The problem is that although Type 1a supernovae are bright, they’re not that bright, universally
speaking.
The most distant supernova ever seen is called UDS10Wil, which detonated about 10 billion
years ago, at a time when the Universe was a third of its current size.
But most have been much closer than that.
In order to get a more precise measurement of the Universe’s early expansion, astronomers
needed another technique, to see those crucial first few billion years.
It looked like they’ve found one, and we’ll get to that in a second, but first I’d like
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On January 29, 2019, a team of astronomers announced that they had accomplished the incredible
feat of figuring out a measurement tool to probe the expansion rate of the Universe during
its first couple of billion years.
They used X-ray observations from NASA’s Chandra X-Ray Observatory and ESA’s XMM-Newton
as well as ultraviolet data from the Sloan Digital Survey to measure about 1,600 quasars.
Quasars, of course, are actively feeding supermassive black holes, and they’re some of the brightest
objects in the Universe.
It turns out that as material piles up in the accretion disk around a black hole, it
produces ultraviolet radiation.
This radiation slams into clouds of hot gas above and below the disk, which boost their
energy and release photons of X-ray radiation.
What this means is that the amount of ultraviolet radiation and X-ray radiation will be a set
ratio to each other.
You can tell how far away a quasar is by measuring the amount of these two kinds of radiation.
Guido Risalti from the University of Florence in Italy and Elisabeta Lusso from Durham University
in the UK carefully used this technique to spot quasars which were as distant as 1.1
billion years after the Big Bang.
This gave them a way to peer into this earlier time of the Universe’s expansion.
They found that, in fact, over the Universe’s 13.8 billion years of history, dark energy
does seem to be changing.
It seems to be increasing.
If true, this would indicate that the future of the Universe could very well include this
Big Rip scenario.
Instead of the stately and solemn heat death of the Universe, countless eons into the future,
we could be looking at a violent end where even black holes are torn to pieces.
One additional bonus is that this discovery could help astronomers with a puzzling paradox:
the expansion rate of the Universe.
Astronomers have two very accurate measurement techniques to measure this rate, one gathered
from the Cosmic Microwave Background, when the Universe was very young, and another measuring
the rate that galaxies are speeding away from each other in all directions.
Changing amounts of dark energy could help close the gap in the two measurements.
Future surveys using other techniques could confirm or invalidate their findings.
We’ll get a better answer over the next decade as telescopes like the Large Synoptic
Survey Telescope, WFIRST and Euclid should be able to get a much more precise answer.
It might turn out that dark energy oscillates, increasing and then decreasing as time goes
on, leading to a long future of acceleration/deceleration cycles, or maybe even a future Big Crunch,
where dark energy turns negative.
It’s important to note that this is a very preliminary result.
There could be all kinds of errors here, in both the underlying data and the techniques
the astronomers used to make their calculations.
In fact, Dr. Paul Sutter mentioned on the Weekly Space Hangout last week that he thought
this discovery probably won’t turn out to be right.
It’s incredibly exciting, so it’s probably wrong.
That’s just how it goes.
Remember primordial gravity waves?
Neutrinos going faster than the speed of light?
Various explanations for dark matter?
They ended up having another explanation.
So, stay tuned.
What do you think?
Let me know your thoughts in the comments.
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