38 | New Scientist | 28 November 2020
matter that warps space-time. Effectively,
Cartan proposed that space-time could also
be affected by the quantum mechanical
property of spin in the matter that makes
up celestial objects.
Torsion is appealing because it is one of the
simplest ways to extend general relativity,
says Christos Tsagas at Aristotle University of
Thessaloniki in Greece. Rather than adding
something ad hoc, you are incorporating a
physical property known to exist in matter.
In the process, you are adding a new field
to the universe, the properties of which are
governed by several parameters that are
still to be constrained.
That gives room for manoeuvre. “You can
fine tune this new field,” says Tsagas. If you
get it right, you can potentially solve the
Hubble tension. “You change the nature of
the geometry of the space-time,” he says, and
anything that affects the geometry of space-
time will affect the expansion of the universe.
Bolejko thinks he might have already done
the trick. “The results that we have are verydistance-duality equation. “This is like a
smoking gun that will be used in future to
test torsion,” says Bolejko.”
As of now, telescopes aren’t sufficiently
sensitive to execute such a test. In the next few
years, however, cosmologists are expecting a
flood of new data. Several upcoming projects
will make huge surveys of the large-scale
structure of the universe, not least the
European Space Agency’s Euclid satellite.
Launching in 2022, Euclid is designed to
measure the shapes and distances of 2 billion
galaxies, with a view to probing the expansion
history of the universe and the formation of
cosmic structures to unprecedented levels
of precision. NASA is planning a similar
mission, the Nancy Grace Roman Space
Telescope, set for launch in the mid-2020s.
That could be make-or-break time for
any challenge to Einstein. “When you start
modifying gravity, you start modifying how
structures like galaxies and galaxy clusters
grow in the universe,” says Mörtsell.
“Satellites like Euclid will be very good at
measuring this with a much higher precision
than now. They will be very useful for
investigating the scenarios where you
actually have another gravitational law
than the one suggested by Einstein.”
Could this really be the end of lambda-
CDM? It might seem impossible that such a
successful model could be felled by a small
discrepancy. Again, history has a different
lesson: it was tiny inconsistencies in things
such as the orbit of the planet Mercury that
set Einstein on the road to replacing Isaac
Newton’s earlier theory of gravity.
Perhaps we are on the cusp of yet another
revolution in cosmology brought about by
fresh observations, even if we don’t know
what that revolution might look like yet.
“To me it’s very exciting,” says Riess,
“because we now have the potential to
discover new things about the universe.” ❚Stuart Clark is a New Scientist
consultant. His latest book is
Beneath the Night (Faber)The European Space
Agency’s Euclid
satellite will measure
billions of galaxies
with peerless precisionencouraging,” he says. “We can actually
explain away the Hubble tension. We are
getting 73.9 [km/s/Mpc]. That’s good enough
for me.” He notes that his work is preliminary.
If his calculations hold up to further scrutiny,
however, it would be the first time in this
argument that a cosmological model has
reproduced the astrophysicists’ value.
One thing in favour of torsion is that there
is an obvious way to test the idea. It involves
comparing two different ways to measure
distance on cosmological scales. One
observes the size of similar celestial objects
and equates any difference to the “angular
distance” between them. The other gets the
distance by comparing the brightness, or
luminosity, of similar objects.
In standard general relativity, those two
distances are related by a specific relationship
known as Etherington’s distance-duality
equation. But earlier this year, Bolejko and
colleagues calculated that with the addition
of torsion, space-time becomes more
complicated and this will change theES
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