36 | New Scientist | 28 November 2020
By this time, astronomers who observed
the rotations of galaxies and clusters of
galaxies had also noted that they are whirling
around far faster than they should be for the
amount of visible matter they contain. The
astronomers’ solution was to update the
model yet again, incorporating a new,
invisible dark matter that far outweighs
the normal stuff we see.
These are the foundations of the standard
model of cosmology, known as lambda-CDM,
the lambda being the dark energy and CDM
standing for “cold dark matter”. It has been
extraordinarily successful, accounting for
pretty much everything we observe in the
universe at its grandest scales. Lambda-CDM
even fits with our most precise map of the
cosmic microwave background (CMB), the
first light in the universe, released just
380,000 years after the big bang. “It is a
perfect model, to be honest,” says Carsten van
der Bruck at the University of Sheffield, UK.Cosmic tension
But that close fit with the CMB suggested a
definitive test of consistency. Cosmologists
could take precise measurements of the
universe’s expansion rate when the CMB was
released and use the model to wind forward
and predict the current rate of expansion,
known as the Hubble constant. “It’s the
ultimate end-to-end test of the universe,”
says Adam Riess, an astrophysicist at Johns
Hopkins University in Maryland. “To go
from the beginning to the end and have
the two ends of the bridge that you are
building meet up.”
The trouble is that they don’t meet. When
we extrapolate forwards from the big bang
using lambda-CDM, we get a lower rate of
expansion than we do through astrophysical
measurements of the distance to exploding
stars in relatively nearby galaxies. The
expansion of the universe is measured as the
speed at which every million parsecs (Mpc) of
space expands, a parsec being 3.26 light years.
Working forward using lambda-CDM,
cosmologists predict a Hubble constant of
68 kilometres per second for every millionparsecs (km/s/Mpc). But looking at the rate of
expansion today by measuring distances in
space, astrophysicists get 73 to 74 km/s/Mpc.
This discrepancy is referred to as the
Hubble tension. If lambda-CDM correctly
describes the universe, it shouldn’t be there.
Most cosmologists, unwilling to give up on
such an otherwise successful model, had
assumed the tension isn’t real – that the
observations were wrong. But last year, a
measurement made using a third method
matched the higher, astrophysics-based value.
This summer, the positions became even
more entrenched when a new look at the CMB
using the Atacama Cosmology Telescope in
Chile bolstered the lambda-CDM prediction.
The message is clear: the measurements
are irreconcilable, and the Hubble tension
is real. There is something fundamental
we don’t understand about the universe.
Over the past year or so, theorists have
been casting around for a fix with fresh
urgency. “It seems like there is a new solution
posed every day,” says Mörtsell. In the grand
tradition of dark energy and dark matter,
many of them involve adding more unseen
ingredients to lambda-CDM in the hope thatNASA
,^ ESA/HUBBL
E,^ HST
FRONTIER^ FIELDSthis will increase the predicted expansion
rate. But when Mörtsell tried to be agnostic
about the nature of an extra ingredient and
just looked at how much energy you would
need to add to the early universe to fix the
tension, the results were sobering. “It is not
easy,” he says. “You can ease the tension a bit.
You can maybe get halfway, but not much
more than that.”
As well as fitting the Hubble constant,
any model must correctly describe other
observations, such as the rate at which
galaxies form, the amount of galaxy
clustering on various cosmological scales
and the appearance of subtle ripples in the
clustering of galaxies, known as baryon
acoustic oscillations. As it stands, lambda-
CDM agrees well with those observations,
and any changes that increase the Hubble
constant quickly put these other predictions
out of whack.
Another option is to tweak the behaviour
of an existing component, for example by
making the repulsive force supplied by dark
energy stronger in the early universe. “You
can ease the tension a bit, but you can’t go
all the way,” says Mörtsell. The same goes