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Last week I told you what dark energy is and why astrophysicists believe it exists. This
week I want to tell you about a recent paper that claims dark energy does not exist.
To briefly remind you, dark energy is what speeds up the expansion of the universe. In
contrast to all other types of matter and energy, dark energy does not dilute if the
universe expands. This means that eventually all the other stuff is more dilute than dark
energy and, therefore, it’s the dark energy that determines the ultimate fate of our universe.
If dark energy is real, the universe will expand faster and faster until all eternity.
If there’s no dark energy, the expansion will slow down instead and it might even reverse,
in which case the universe will collapse back to a point.
I don’t know about you, but I would like to know what’s going to happen with our
universe.
So what do we know about dark energy. The most important evidence we have for the existence
of dark energy comes from supernova redshifts. Saul Perlmutter and Adam Riess won a Nobel
Prize for this observation in 2011. It’s *this Nobel-prize winning discovery which
the new paper calls into question. You find the reference in the information below the
video.
Supernovae give us information about dark energy because some of them are very regular.
These are the so-called type Ia supernovae. Astrophysicists understand quite well how
these supernovae happen. They are produced by white dwarfs colliding with normal stars.
This allows physicists to calculate how much light these blasts emit as a function of time,
so they know what was emitted. But the farther the supernova is away, the dimmer it appears.
So, if you observe one of these supernova, you can infer its distance from the brightness.
At the same time, you can also determine the color of the light. Now, and this is the important
point, this light from the supernova will stretch if space expands while the light travels
from the supernova to us. This means that the wave-lengths we observe here on earth
are longer than they were at emission or, to put it differently, the light arrives here
with a frequency that is shifted to the red. This red-shift of the light therefore tells
us something about the expansion of the universe.
Now, the farther away a supernova is, the longer it takes the light to reach us, and
the longer ago the supernova must have happened. This means that if you measure supernovae
at different distances, they really happened at different times, and you know how the expansion
of space changes with time.
And this is, in a nutshell, what Perlmutter and Riess did. They used the distance inferred
from the brightness and the redshift of type 1a supernovae, and found that the only way
to explain both types of measurements is that the expansion of the universe is getting faster.
And this means that dark energy must exist.
Now, Perlmutter and Riess did their analysis 20 years ago and they used a fairly small
sample of about 110 supernovae. Meanwhile, we have data for more than 1000 supernovae.
For the new paper, the researchers used 740 supernovae from the JLA catalogue. But they
also explain that if one just uses the data from this catalogue as it is, one gets a wrong
result. The reason is that the data has been “corrected” already.
This correction is made because the story that I just told you about the redshift is
more complicated than I made it sound. That’s because the frequency of light from a distant
source can also shift just because our galaxy moves relative to the source. More generally,
both our galaxy and the source move relative to the average restframe of stuff in the universe.
And it is this latter frame that one wants to make a statement about when it comes to
the expansion of the universe.
How do you even make such a correction? Well, you need to have some information about the
motion of our galaxy from observations other than supernovae. You can do that by relying
on regularities in the emission of light from galaxies and galaxy clusters. This allow astrophysicist
to create a map with the velocities of galaxies around us, called the “bulk flow” . But
the details don’t matter all that much. To understand this new paper you only need
to know that the authors had to go and reverse this correction to get the original data.
And *then they fitted the original data rather than using data that were, basically, assumed
to converge to the cosmological average.
What they found is that the best fit to the data is that the redshift of supernovae is
not the same in all directions, but that it depends on the direction. This direction is
aligned with the direction in which we move through the cosmic microwave background. And
– most importantly – you do not need further redshift to explain the observations.
If what they say is correct, then it is unnecessary to postulate dark energy which means that
the expansion of the universe might not speed up after all.
Why didn’t Perlmutter and Riess come to this conclusions? They could not, because
the supernovae that they looked at all came from the same direction of the sky, and that
direction happens to be pretty much opposite to the CMB dipole. And if you look only in
one direction, you can’t tell if the effect you see is the same in all directions.
What is with the other evidence for dark energy? Well, all the other evidence for dark energy
is not evidence for dark energy in particular, but for a certain combination of parameters
in the concordance model of cosmology. These parameters include, among other things, the
amount of dark matter, the amount of normal matter, and the Hubble rate.
There is for example the data from baryon acoustic oscillations and from the cosmic
microwave background which are currently best fit by the presence of dark energy. But if
the new paper is correct, then the current best-fit parameters for those other measurements
no longer agree with those of the supernovae measurements. This does not mean that the
new paper is wrong. It means that one has to re-analyze the complete set of data to
find out what is overall the combination of parameters that makes the best fit.
This paper, I have to emphasize, has been peer reviewed, is published in a high quality
journal, and the analysis meets the current scientific standard of the field. It is not
a result that can be easily dismissed and it deserves to be taken very seriously, especially
because it calls into question a Nobel Prize winning discovery. This analysis has of course
to be checked by other groups and I am sure we will hear about this again, so stay tuned.