In 1929, astronomer Edwin Hubble noticed that almost all galaxies had a red shift to their
color, and the farther away they were, the redder they appeared.
He concluded that the waves of light from the galaxies had been stretched as they traveled
to us, meaning space itself was expanding in all directions.
This observation literally shaped our image of the universe, but now some researchers
think that shape may be a bit lopsided.
The idea that space looks pretty much the same everywhere is known as the cosmological
principle.
Sure, close up there are variations.
The structure of star systems, galaxies, or even galaxy clusters are going to be different.
But on an extremely zoomed-out scale of many billions of light years, it’s believed the
universe looks and behaves the same way in every direction.
In other words, it’s isotropic.
We think it started with a big bang 13.8 billion years ago, and like a balloon, has expanded
more or less evenly in every direction since.
A good piece of evidence supporting the notion that the universe is isotropic is the cosmic
microwave background, the afterglow left by the big bang, which looks pretty smooth and
uniform.
An isotropic universe is convenient.
If everything is behaving the same and expansion is happening evenly, then we can assume dark
energy, what we think is driving the accelerating expansion, acts the same everywhere, and has
been since the big bang.
But what if somewhere along the way the rules changed, and the universe isn’t expanding
evenly anymore?
Some astronomers decided to double check using X-ray emissions from galaxy clusters, and
have concluded that the universe may not be as isotropic as we thought, in other words,
it could be “anisotropic.”
The researchers looked at data from almost 850 galaxy clusters.
Using their X-ray emissions, the astronomers estimated each cluster’s temperature, and
thus its luminosity.
Keep in mind that luminosity is how much light a celestial object gives off, meaning it’s
an intrinsic property and should be the same no matter how you measure it.
Once they had used x-ray observations to determine the clusters’ luminosities, they cross-checked
the numbers using another method that relies on the rate we assume the universe is uniformly
expanding.
When they compared the two values, they found two areas of the night sky where the numbers
didn’t match up.
In these regions, the clusters were as much as 30% brighter or dimmer than predicted,
meaning they were closer or farther away than they should be if the universe were isotropic.
That would throw a serious wrench into our understanding of the cosmos.
Why would the cosmic microwave background be so smooth when the recent universe isn’t?
Would this mean the laws of physics are different in certain areas?
Is it possible dark energy affects some regions more strongly than others?
It’s hard enough finding an explanation for dark energy when it behaves uniformly.
These are the questions an anisotropic universe would raise, and their answers may be unknowable.
But one possibility suggested by one study doesn't overturn all of our assumptions about
isotropy.
These researchers are looking to explain the findings in ways that gel with the cosmological
principle.
One explanation is some clusters could be caught in a “bulk flow” where even more
massive clusters farther away drag them off.
Or there’s the possibility that the data the conclusions are based on are just plain
wrong.
One astronomer pointed out that one region the researchers deemed particularly lopsided
is also near an area where the Milky Way’s gas and dust are thickest.
It’s possible that’s absorbing x-rays and throwing off the luminosity calculations.
But the researchers do take the effect of the Milky Way into account.
Even if their results were due to an absorption effect, it would mean some new kind of super-dust
was scrambling the emissions (which gives me serious Golden Compass vibes).
The researchers intend to follow up with infrared observations to take another
stab at determining the clusters’ luminosities.
Maybe their future results will show that their assumptions based on x-rays were off
and the universe is the same in all directions.
Then again, maybe their results will change everything we know about the history and shape
of the cosmos.
If ever a topic needed more research, this is it.
One instrument that will help map out the dark energy of the universe and answer this
riddle is ESA’s Euclid telescope.
Amanda has a Countdown To Launch episode on that here.
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