Why is there life in our
universe? The existence of galaxies, stars, planets, and ultimately life seem
to depend on a small number of finely tuned fundamental physical constants. Had
the laws of physics been different, we would not have been around to debate the
question. So how come the laws of our universe just happen to be the way they
are—is it all a lucky coincidence?
In the last few decades, an
increasingly popular theory has come to the fore. The multiverse theory
suggests that our universe is just one of many in an infinite multiverse where
new universes are constantly being born. It seems likely that baby universes
are produced with a wide range of physical laws and fundamental constants, but
that only a tiny fraction of these are hospitable for life. It would therefore
make sense that there is a universe with the strange fundamental constants we
see, finely tuned to be hospitable for life.
But now our new discovery,
published in the Monthly Notices of the Royal Astronomical Society, complicates
things by suggesting that life may actually be a lot more common in parallel
universes than we had thought.
Our universe started with a
Big Bang, followed by a period of very rapid expansion, known as inflation.
However, according to modern physics, inflation is unlikely to have been a
single event. Instead, many different patches of the cosmos could suddenly
start inflating and expand to huge volumes—each bubble creating a universe in its
own right.
Some believe that we may one
day be able to witness imprints of collisions with parallel universes in the
cosmic microwave background, which is the radiation left over from the birth of
the universe. Others, however, believe the multiverse is a mathematical quirk
rather than a reality.
One hugely important
constant in the universe is a mysterious, unknown force dubbed dark energy. At
the present day, this makes up 70% of our universe. Rather than our universe
slowing down as it expands, dark energy causes its expansion to accelerate.
But many current theories
suggest that dark energy should be much more plentiful than this across the
multiverse. Most universes should have an abundance of dark energy that is
around a million, billion, billion, billion, billion, billion, billion times
larger than in our universe. But if dark energy were this abundant, the
universe would rip itself apart before gravity could bring together matter to
form galaxies, stars, planets or people.
While our universe has a
strangely low value of dark energy, it is this low value that makes our
universe hospitable to life. The multiverse theory can help us explain why it
is so low—there will always be some universes with unlikely values in an
infinite multiverse.
This is because larger
values of dark energy should be more common in the multiverse than lower
values. At the same time, we expect life to exist only in a small group of
universes with a value below a certain maximum—those in which matter can still
clump together to form stars and galaxies. This means that universes with a
comparatively high value of dark energy (close to maximum) that are hospitable
to life should be more numerous than universes with low values (close to
minimum), meaning they are more likely.
So do we live in such a
universe? Through our study, we set out to find out what this maximum level is
and whether we are close to it.
The simulations take the
laws of physics and follow the formation of stars and galaxies as the universe
expands after the Big Bang. The galaxies that emerge in our model look
remarkably like those seen in the night sky through telescopes. Each simulation
led to a universe with specific structure.
This success makes it
possible to convincingly investigate how the formation of stars and galaxies
would proceed in other parts of the multiverse. We created a series of
computer-generated universes that were identical apart from having different
amounts of dark energy. Initially, the universes all expanded at similar rates
but, as the energy left over from the Big Bang dissipated, the power of dark
energy became important. The universes with abundant dark energy accelerated
vigorously.
That means our own universe
does not have a value of dark energy that is close to the maximum for life to
exist. The effects of gravity are much more robust than we had previously
thought. Life, it seems, would be rather common throughout the multiverse,
perhaps a million, billion, billion, billion, billion more common that we previously
thought.
Our discovery puts the idea
that an infinite multiverse can explain the low abundance of dark energy on
very rocky ground. Interestingly, in his last published paper, Stephen Hawking
argued that the multiverse is far from infinite, and that it is more likely to
contain a finite number of rather similar parallel universes.
It seems that a new physical
law, or a new approach to understanding dark energy, is needed to account for
the deeply puzzling properties of our universe. But the good news is that we
are one step closer to cracking it.
This article was originally
published on The Conversation. Read the original article.
No comments