WENDY 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 there may 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 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.
Northern winter constellations and a long arc of the Milky Way are setting
in this night skyscape looking toward the Pacific Ocean from Point Reyes on
planet Earth's California coast. Sirius, alpha star of Canis Major, is
prominent below the starry arc toward the left. Orion's yellowish
Betelgeuse, Aldebaran in Taurus, and the blue tinted Pleiades star cluster
also find themselves between Milky Way and northwestern horizon near the
center of the scene. The nebulae visible in the series of exposures used to
construct this panoramic view were captured in early March, but are just too
faint to be seen with the unaided eye. On that northern night their
expansive glow includes the reddish semi-circle of Barnard's Loop in Orion
and NGC 1499 above and right of the Pleiades, also known as the California
Nebula.
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. In 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
measurements from 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 explain how the two values could be different. “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. Tip of the red giant branch
stars are simpler than Cepheids, and we have a better understanding of the
physics that determines how bright they are and their colours. They are also
extremely common and located throughout galaxies, whereas Cepheids are
generally more concentrated towards the centres. This means we don’t have to
worry too much about other stars or dust contaminating our images, as we can
simply look at stars that are in less busy areas.
These stars have been used to measure distances for a long time, but they
fell out of favour because they are dimmer than Cepheids. Now that
telescopes have improved, the simpler stars are coming back to the fore.
“This method has always been there, sort of looming in the background,” says
Barry Madore at the Carnegie Institution for Science in California,
Freedman’s husband and frequent collaborator. “Its simplicity is just
crushingly, mind-blowingly good. There are so few things that can go wrong
with it.”
So when Freedman’s most recent measurements using the tip of the red giant
method yielded a Hubble constant of 69.8 – right between the CMB and the
Cepheid numbers – it sowed fresh seeds of doubt among the growing certainty
in the community that believe in the Hubble tension. “It was quite a
surprise that she came up with these results that are in between, and sort
of mitigated the growing feeling that we had that there really was an
issue,” says Brent Tully at the Institute for Astronomy in Hawaii. “I think
that Wendy makes a good case that there are still outstanding problems.”
Once again, Freedman finds herself at the centre of the debate. “We imagined
that when we got our result that it would land on one side or the other, but
it just didn’t,” says Freedman. “It’s saying this isn’t really tied up yet.”
Not everyone agrees – Riess in particular still claims that it actually is
tied up, and the Hubble tension is real – but Freedman remains cautious.
And once again, the solution may come from a huge new space telescope. This
time, it is the James Webb Space Telescope (JWST), which launched at the end
of 2021 and is expected to begin observations around July this year.
The JWST has a larger mirror than Hubble, so it can look at more distant
objects, and unlike Hubble, the JWST is an infrared telescope. That isn’t
ideal for Cepheids, which are relatively blue in colour. It will help us
improve the precision of our current Cepheid observations, but it is
unlikely we will find any more distant ones. The tip of the red giant branch
is a different story. As their name suggests, these stars are red, so when
viewed in the infrared, they appear brighter than the surrounding stars. “I
think that we’re really gonna nail this thing through the tip of the red
giant branch,” says Tully.
Freedman has been approved to observe those stars with the JWST once it
starts up, and in the meantime she is observing them and Cepheids from the
ground, as well as looking into a potential new method using stars that are
extremely rich in carbon. “If you knew the answer, you’d stop,” says Madore.
“We don’t know the answer.”
The solution to the Hubble tension may come from measurements, but it will
be deeply entangled in theoretical calculations about the cosmos as well.
“You can’t just measure your way to an answer – these things go hand in
hand,” says Riess. If the tension isn’t real, we need to figure out what we
misunderstood in the first place. If it is real, there is a far deeper well
of misunderstandings. Should we learn that our standard model of the
universe is incomplete, we will doubtless find other places where that model
breaks down.
“If the Hubble anomaly is real, then there have to be other anomalies,” says
Peebles. To build a better model of the cosmos, we will have to hunt each
anomaly down, prove it is real and figure out where it came from. All this
means more hard work, and more waiting. But Freedman doesn’t mind – it’s
business as usual for her. “The universe is doing whatever it’s doing,” she
says. “It doesn’t care when, or whether, we answer our questions about it.”
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