14 May 2022 | New Scientist | 47Features
coming in from her new observations,
they indicated a Hubble constant closer to
80, contradicting two of the most renowned
cosmologists in the world. Sandage wasn’t
thrilled, to say the least. “It’s hard being
contradicted by a young upstart – what do they
know?” says Peebles. “Well, Wendy knew a lot.
She is an absolute model of toughness.”
You might think it would be a scary place
to be, going against the most prominent
astronomers of the time. “I didn’t find it scary,”
says Freedman. “It was fun.” She knew her
results were clear. Her calculations used
observations of Cepheids, stars that pulsate
regularly. Sandage and de Vaucouleurs used
these stars, too. But crucially, Freedman’s
early observations were among the first to be
corrected to factor in the dust between us and
the Cepheids, making the calculated distances
more accurate than those in previous work.
Cepheids have a pulsation period that is
directly related to their absolute luminosity,
so comparing that with how bright they
appear from Earth can tell us how distant they
really are. Those measurements can be used to
extrapolate outwards, using supernovae in the
same galaxies as the Cepheids. This method is
known as the cosmic distance ladder, because
each step along the way builds up to the next.
Over the decades since her initial results,
Freedman’s measurements have held up.
Cepheids have remained the main tool by
which we measure the expansion of the
universe in the area relatively close to Earth –
one of the first steps on the distance ladder.Distance unknown
Physicists have been arguing over the speed of the
universe’s expansion for decades – but have they been
measuring space wrong all along, asks Leah Crane
W
ENDY FREEDMAN is staring down
the universe. For 40 years, she has
been digging into the biggest
secrets of the cosmos, patiently whittling down
uncertainties to find the value of a number
that defines the expansion of the universe,
determines its age and seals its ultimate fate.
Freedman, who works at the University
of Chicago, studies the Hubble constant,
a number that represents how fast the
expansion of the universe is accelerating. We
have known about this escalating expansion
since 1929, when US astronomer Edwin Hubble
found that the more distant an object was, the
faster it seemed to be moving away from us.
That is when things got tricky. Pinning down
the numbers requires accurate measurements
of astronomical distances. In Hubble’s era,
astronomical images were taken by shining
light through a telescope onto a photographic
plate. Calculating distances from those images
was difficult and imprecise.
In the 1980s, as Freedman was finishing
her PhD, digital photography was getting
ready to revolutionise astronomy as a whole,
and measurements of the Hubble constant
in particular. “That’s really what spurred me,”
says Freedman. In the decades since, her
work has been key to the development of
the Hubble tension – the perplexing way
that the two main ways of measuring the
Hubble constant give us different values.
Now, after Freedman has spent decades
focusing on this problem, something curious
is happening. Her newest results suggest theremay be no problem after all. If this is the
case, it will render pointless decades of work
exploring new physics that could explain the
discrepancy. Luckily, Freedman isn’t afraid
of a little controversy.
The Hubble constant is a big piece of
the cosmological jigsaw that, when put
together, tells us about the history and future
of the universe. If we know how quickly the
expansion of the universe is accelerating,
that hints how big the cosmos is, how old it is
and how it began. Looking ahead, the Hubble
constant determines whether the universe
will expand forever or collapse in a big crunch.A cosmic ladder
Before Freedman came on the scene, there
were two main estimates of the Hubble
constant. French astronomer Gérard de
Vaucouleurs found it to be 100 kilometres
per second per megaparsec – a megaparsec
being equal to 3.26 million light years. But US
astronomer Allan Sandage found it to be much
lower, at about 50 km/sec/mpc. The two were
locked in a fierce debate that had been raging
for decades, until the 1980s. At that time,
Freedman was a postdoctoral fellow at the
Carnegie Observatories in California, where
Sandage was a professor. Although well-
respected, Sandage was sometimes feared for
his anger. He would “go non-linear” when he
was contradicted, says Princeton University
cosmologist and Nobel laureate Jim Peebles.
When Freedman’s first results started >