14 May 2022 | New Scientist | 49it 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.” ❚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 more
interesting
to say there
is new physics,
but that doesn’t
mean there is”
Leah Crane is a space and physics
reporter for New Scientist. Follow
her on Twitter @DownHereOnEarthof 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
Certain stars
help us work out
the distances to
galaxies beyond
the Milky Way