V W PICS / UNIVERSAL IMAGESIt’s often said that science is self-
correcting. First, a scientist presents a
result, openly admitting that they may
have made a mistake somewhere along the
way. Next, other scientists step in to check
their numbers, reproduce the experiment,
or, better yet, test the conclusion via an
entirely different, independent method. If
all parties agree, we have a consensus, and
people begin to cautiously trust the original
conclusion.
It doesn’t stop there, of course –
anything we think we know in science can
change based on new data or analyses – but
if you’re going to rank the believability of
different kinds of results, those supported
by more than one group in more than one
way should be highest on your list.
The tricky part is when experiments
disagree. Hard science should never be a
one method, we observe a large number
of galaxies around us and measure how
quickly they seem to be receding. The other
is based on looking at the most distant light
in the Universe – the cosmic microwave
background – and carefully measuring the
sizes of hotter and colder spots in that light
to conclude something about the shape
of space and the whole history of cosmic
expansion.
While the galaxy method might seem
far more straightforward, it’s complicated
by the fact that it requires us to know the
distances to those galaxies very precisely.
And because we figure that out by observing
stellar explosions, it requires us to have an
extremely good understanding of how stars
work, in a range of galactic environments.
Astronomers have always expected
both methods to give us the same answer: amatter of opinion; it’s all based on the idea
that there is a right and replicable answer,
and through rigorous experimentation
and logical thought, we will find it. Any
disagreement about some fundamental
property of the natural world has to be a
mistake, and if we retrace our steps carefully
enough, we’ll find it.
Or, alternatively, the whole framework
on which we’ve based our understanding of
the experiments is wrong.
This is the dilemma we’re facing now in
cosmology. It concerns what seems like a
very straightforward question: How quickly
is the Universe expanding?
In the last few years, it’s become
clear that two of the methods previously
considered to be the most precise and
reliable ways to measure this have been
pointing us toward different answers. In24 – COSMOS Issue 86
DIGEST
Hubble trouble,
boils and bubbles
How quickly is the Universe expanding?
It’s a question currently causing a crisis in
astrophysics – and as Katie Mackexplains,
that’s a wonderful problem to have.