Travel far enough
in the universe and you could end up back where you began. Measurements from
the Planck space observatory have shown that the universe might be shaped like
a sphere rather than a flat sheet, which would change nearly everything we
think we know about the cosmos.
The Planck
observatory, which operated from 2009 to 2013, mapped the
cosmic microwave background, a sea of light left over from the big
bang.
One set of
observations showed that there was more gravitational lensing – stretching of
the light due to the shape of space-time, which can be distorted by heavy
matter – than expected. Alessandro Melchiorri at the Sapienza University of
Rome and his colleagues calculated that this could be because the shape of the
universe is different from what we thought.
All other
cosmological data suggests the universe is flat, meaning it has no curvature,
similar to a sheet of paper. These Planck measurements indicate that it could
be “closed”, or spherical, which would mean that if you travelled far enough in
one direction, you would end up back where you started. That is because the
extra lensing implies the presence of extra dark matter, which would pull the
universe into a finite sphere instead of a flat sheet.
According to these
observations, the universe is 41 times more likely to be closed than flat.
“This is the most precise cosmological data and it is giving us a different
picture,” says Melchiorri.
If the universe is
indeed closed, that could be a major problem for our understanding of the
cosmos. Another cosmological puzzle is that the nearby universe seems to
be expanding
faster than it ought to. This is tough to explain with our
standard model of cosmology, which includes a flat universe, and the team
calculated that this gets even tougher with a spherical universe, along with a
few other cosmic mismatches we have yet to explain. It is so bad that they are
calling it a “cosmological crisis”.
“In a closed
universe, these anomalies are more serious than we thought,” says Melchiorri.
“If nothing is in agreement, we have to think hard about our model of the
universe and its formation.”
The usual
explanation of the universe’s formation includes a period just after the big bang called
inflation, when the universe rapidly expanded. Our current models of inflation
naturally lead to a flat universe, so if the universe is actually closed, they
would have to change.
“We need a new
model, and we don’t know what that is yet,” says Melchiorri. Nobody has come up
with a way to reconcile these Planck observations with the many cosmological
measurements that disagree, which even include some of the other observations
from the Planck observatory.
In fact, every
other cosmological measurement that we have points to a flat universe. There
are no other observations that hint the cosmos may actually be closed, and
there is a chance that this Planck measurement is just a statistical fluke.
“If this is true,
it would have profound implications on our understanding of the universe,” says
David Spergel at Princeton University. “It’s a really important claim, but I’m
not sure it’s one that’s backed by the data. In fact, I’d say the evidence is
actually against it.”
More data in the
next few years will show whether we need to take this anomaly seriously or if
it is simply a statistical fluke, says Spergel. The Simons Observatory, which
is currently being built in Chile, will be able to measure gravitational
lensing even more precisely than Planck, and it should tell us whether or not
there really is a cosmological crisis.
Journal reference: Nature Astronomy, DOI:
10.1038/s41550-019-0906-9
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