When we look into the night sky, we see the universe as it once was. We know
that in the past, the universe was once warmer and denser than it is now.
When we look deep enough into the sky, we see the microwave remnant of the
big bang known as the cosmic microwave background. That marks the limit of
what we can see. It marks the extent of the observable universe from our
vantage point.
The cosmic background we observe comes from a time when the universe was
already about 380,000 years old. We can't directly observe what happened
before that. Much of the earlier period is fairly well understood given what
we know about physics, but the earliest moments of the big bang remain a bit
of a mystery. According to the standard model, the earliest moments of the
universe were so hot and dense that even the fundamental forces of the
universe acted differently than they do now. To better understand the big
bang, we need to better understand these forces.
One of the more difficult forces to understand is the weak force. Unlike
more familiar forces such as gravity and electromagnetism, the weak is
mostly seen through its effect of radioactive decay. So we can study the
weak force by measuring the rate at which things decay. But there's a
problem when it comes to neutrons.
Together with protons, neutrons make up the nuclei of the atoms we see
around us. Within an atomic nucleus, neutrons can be extremely stable. But
when a neutron is on its own, it typically decays in a matter of minutes.
The rate of decay for neutrons is typically given in terms of its half life.
That is, the time at which a neutron has about a 50/50 chance of having
decayed. Technically, they measure a related quantity known as the neutron
lifetime, but the idea is the same.
There are a couple of ways we can measure neutron half-life, such as
measuring a beam of neutrons or cooling them down and trapping them in a
magnetic bottle, but these different methods give different results for the
half-life. The methods should give the same result, but they don't. The beam
method gives a lifetime of 888 seconds, while the bottle method gives 879
seconds. Perhaps there is some systematic error in the methods, but this
discrepancy is a problem for fundamental physics. But a new study has
measured neutron decay in a third way, by using a spacecraft orbiting the
moon.
The airless surface of the moon is constantly bombarded by cosmic rays.
Sometimes a cosmic ray will kick a neutron off the lunar surface. As the
neutron speeds away from the moon, it has a chance of decaying. So the team
used NASA's Lunar Prospector satellite to count the number of neutrons at
various orbital heights. From this, they calculated the neutron lifetime to
be 887 seconds.
The result isn't precise enough to resolve the neutron decay problem, but it
does show that we can use spacecraft to get very accurate results. Accurate
enough that future missions might be able to solve the weakest link of early
cosmology.
Reference:
Jack T. Wilson et al, Measurement of the Free Neutron Lifetime using the
Neutron Spectrometer on NASA's Lunar Prospector Mission. arXiv:2011.07061v2
[nucl-ex],
arxiv.org/abs/2011.07061
Tags:
Space & Astrophysics