48 | New Scientist | 14 May 2022In fact, observing them, and measuring the
Hubble constant to 10 per cent accuracy, was
one of four so-called key projects of the Hubble
Space Telescope, launched in 1990. “The director
of the project asked, if Hubble fell into the
ocean a month after it started observing, what
were the projects we’d really want done,” says
Freedman. “Every telescope has a big project
that’s its goal, and settling this debate was the
big problem that was sort of before us.”
The project was a resounding success. Using
the telescope, Freedman’s team measured a
Hubble constant of about 72 km/sec/mpc –
right in between the earlier estimates. Even as
instrumentation has improved since the key
project paper was published in 2001, the value
we get from measuring Cepheids has remained
similar, with the most recent observations
putting it around 73 km/sec/mpc.
But that isn’t the end of the story, because
Cepheids aren’t the only way to measure the
Hubble constant. Another problem came along
when astrophysicists started observing the
cosmic microwave background (CMB), relic
light left over from the moments after the big
bang. By observing this light and extrapolating
forwards in time based on our best models of
the universe, we can predict what the Hubble
constant ought to be today.
“The CMB is really predicting the Hubble
constant starting at the other end of the
universe, using our story of cosmology and
physics,” says Adam Riess at Johns Hopkins
University in Maryland, who won a Nobel prize
for his work on the expansion of the universe.
And these measurements from the other end
of the universe, which are extraordinarily
precise, don’t agree with the measurementsfrom this end. They put the Hubble constant
at around 67 km/sec/mpc.
While the CMB measurements themselves
are extremely precise, the value of the Hubble
constant we get from them is calculated using
physicists’ standard model of the cosmos – a
set of equations that fit just about everything,
from general relativity to the effects of dark
matter. “I continue to be astounded at how
well theory and observation fit,” says Peebles.
“But eventually, we’re going to find something
that doesn’t fit, and maybe this is it.”Tension or no tension?
If both the Cepheid measurements and
the CMB measurements are correct,
a problem known as the Hubble tension, then
something is wrong with our understanding
of the cosmos. And if we shift one part of
that understanding – for instance, the way
the universe inflated after the big bang – it
won’t just solve the Hubble tension. It will
have a knock-on effect on other factors, in
ways we don’t know how to account for.
“If anything changes, it’s going to change
everything,” says Freedman.
The big question now is whether it is time
to change everything. Riess claims that it is.
He says the CMB measurements are solid. His
group has made many Cepheid measurements,
and the tension between the two methods
remains. “It’s hard to ignore that basically
all the precise measurements are coming
in higher than the CMB,” he says.
Other cosmologists aren’t so convinced,
not least because of how difficult it is to
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“People are trying to come up with ways to
explain this, and almost 1000 papers later,
they haven’t,” says Freedman. “It’s much more
interesting to say there’s new physics than
there are systematic uncertainties, but that
doesn’t mean it’s how the universe is.”
Nailing down whether the Hubble
tension is real needs more measurement
methods, says Freedman. Relying solely on
Cepheids, it is impossible to know if we are
misunderstanding those stars in some basic
way that throws measurements off. The
uncertainties in the CMB measurement are
below 1 per cent. Ideally, uncertainties in
measurements of the local Hubble constant
would be similarly low. “Right now, there’s
disagreement about the errors of the errors,”
says Freedman – it isn’t clear where exactly the
uncertainties are now, but she doesn’t believe
we are at 1 per cent yet. “I think to be certain
that the errors are less than 1 per cent, you
need more than one type of measurement,
more than one instrument, more than one
technique,” she says. “I just think we need
to be a little bit patient.”
That is why Freedman turned to a different
source, called tip of the red giant branch stars.
These are the brightest stars in a group called
the red giants, which make up a branch on the
Hertzsprung-Russell diagram, a plot of stars’
temperature against their luminosity. TipMeasuring the
Hubble constant
was a key project
for the Hubble
Space Telescope