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| Artist’s rendition of a concentration of black holes: ESA/Hubble, N. Bartmann. |
Primordial black holes older than the big bang could rewrite cosmology by
providing evidence for a previous universe. It's a wild idea, but some
physicists think we've got a chance of finding them.
ALMOST exactly 50 years ago, when I was a PhD student, I wrote an article in
this magazine about the mounting evidence for black holes, regions of space
in which gravity is so strong that light can never escape. Today, there is
no longer any doubt about their existence. We know they form from collapsing
stars and that supermassive ones sit at the centres of galaxies. We have
even taken a picture of two of them. But in my article I also mentioned a
more speculative possibility: that smaller black holes might have formed in
the early universe, shortly after the big bang.
I was working on this idea under the supervision of Stephen Hawking, who had
started to think about such a possibility just a few years earlier. Our work
together set the trajectory of my career, much of which has been dedicated
to studying what we now call primordial black holes. We still don’t know if
they formed, but there are good reasons to think they might have. Some of
them could still be around today and, excitingly, they could be the answer
to a whole range of cosmological conundrums.
Recently, however, I have become interested in an even more exotic
possibility: that some black holes could be older than the universe itself.
It is a wild idea, but not inconceivable. And new research suggests that we
might one day be able to positively identify them, a breakthrough that would
radically change our understanding of cosmology.
Most cosmologists would claim that all the matter and energy that permeates
our universe today came into existence in a single moment 13.8 billion years
ago that we call the big bang. After that, there was a period when the
universe grew exponentially fast, called cosmic inflation, before it settled
down to a gentler expansion.
The big bang
One problem with this picture is that we don’t know for sure what happened
at the big bang. It is often described as a singularity – a point of
infinite density – and Albert Einstein’s general relativity, our best
description of gravity, breaks down at a singularity. As a result, we can’t
describe it with the usual equations that make sense of reality.
This has led some cosmologists to speculate that the universe started with a
big bounce. Instead of everything springing into existence in one moment, a
big bounce would be the result of a previous, collapsing universe starting
to expand again. This is a kind of big bang, but without a singularity, as
the universe always has a finite density. The bouncing scenario is
compatible with certain attempts at uniting the laws of physics, such as
some models of quantum cosmology, loop quantum gravity and some alternative
theories of gravity.
If our universe came from a bounce, it might end in one too. This kind of
recurring bounce, where the universe goes through periods of expansion and
compression, is called a cyclic universe. It only applies if the universe is
destined to recollapse and this, in turn, depends on the nature of dark
energy, the mysterious force causing the universe to fly apart faster and
faster. Nevertheless, if we found evidence for these bouncing and cyclic
models, it would have huge implications, both for how the universe began and
how it might end.
Finding this evidence is tricky, partly because everything that would have
existed in the previous universe is likely to have been destroyed when it
collapsed. Or would it? I think there is a chance that some black holes from
a previous universe may have survived the big bounce and still be around
today.
The idea that black holes may have formed in the early universe dates to the
early 1970s. Stephen and I had been considering whether black holes could
form from density fluctuations near the big bang. From our calculations,
that did indeed seem possible. But there was a snag. A few years earlier,
Russian researchers Yakov Zeldovich and Igor Novikov had shown any black
holes formed in the early universe would grow rapidly, reaching an enormous
mass today. This was ruled out with observations – we would have seen the
effects of such black holes, so they concluded that primordial black holes
never formed.
My first paper with Stephen showed this result was wrong. After many days of
calculation, I rushed excitedly to his office to give the good news: because
of the expansion of the universe, which the pair hadn’t considered,
primordial black holes wouldn’t grow much at all. I was rather deflated to
find Stephen had just come to the same conclusion, independently, by doing
the calculation in his head. Nevertheless, we agreed: primordial black holes
may have existed, after all.
Primordial black holes
Fifty years later, we still haven’t seen any of these black holes for
certain, although some people think there are hints of them in detections of
ripples in space-time called gravitational waves. What we can say for
certain, at least, is that thinking about them prompted Stephen to discover
the radiation that black holes give off, which we call Hawking radiation,
and the black hole information paradox (see “Black hole candy”).
Of course, it would be much more interesting if primordial black holes did
form, and in recent years there has been a growing interest in the idea. We
know that any black holes weighing in at less than 1 trillion kilograms,
roughly the mass of a mountain, but the size of a proton, would have
evaporated by now because of Hawking radiation. But any black holes bigger
than that would exist today.
However, there is an even more intriguing possibility. Around 10 years ago,
Alan Coley at Dalhousie University in Halifax, Canada, and I became
interested in whether we live in a cyclic universe. We started to consider
whether black holes might have formed in a previous cosmic cycle and
realised there were two possibilities.
The first is that they formed due to the high density of the previous
universe in the final moments of its collapse. This “big crunch” is just
like the high-density phase in the big bang, but running backwards in time.
So if black holes can form in the big bang, they might also form in the big
crunch. In this case, they would have a minimum mass determined by the
density of the universe at the bounce, the time at which the universe is at
its most dense. If this density is small enough, the black holes could be
large enough to potentially explain dark matter, the mysterious stuff that
keeps galaxies from flying apart, or the origins of supermassive black
holes.
Later work investigated this in more detail. In 2016, Jerome Quintin and
Robert Brandenberger, both at McGill University in Montreal, Canada,
calculated the quantum and thermal fluctuations of a collapsing universe.
They found that black holes can indeed form, albeit only if the universe is
dominated by matter, not radiation.
The second possibility is that black holes formed in an earlier phase of the
previous universe – just like the black holes that form from the collapse of
stars or galactic nuclei in our universe. In either case, our next question
was whether the pre-big bang black holes would survive the bounce and
persist into the current cycle. This depends on the fraction of the volume
of the universe occupied by black holes at the bounce. We reasoned that one
could expect black holes to persist if their separation at the bounce was
greater than their typical size, because they wouldn’t be squeezed together
and merge. We concluded that this should be possible in many situations.
In 2015, Timothy Clifton, my colleague at Queen Mary University of London,
along with Coley and myself, made a stab at tackling this question in a more
mathematically rigorous way. We derived some exact solutions to Einstein’s
equations of general relativity, describing a regular lattice of black holes
in a universe that undergoes a bounce. Our results indicated there are
indeed solutions in which multiple black holes persist through a bounce.
Inflation not required
Later, we also looked into some cosmological consequences of this proposal,
arguing that pre-big bang black holes in different mass ranges could explain
dark matter, provide seeds for galaxies and perhaps even cause the bounce
itself. In the standard big bang scenario, primordial black holes generated
before inflation would be exponentially diluted, so any present today are
usually assumed to form after inflation, but there is no inflation in some
bouncing models.
Other researchers subsequently elaborated on those ideas. In 2018, Carlo
Rovelli at Aix-Marseille University in France and Francesca Vidotto at
Western University in Ontario, Canada, investigated the possibility that
dark matter is made up of the remnants of pre-big bang black holes. They
argued only a tiny fraction of the volume of the universe would be outside
these black holes at the bounce, though observers in these regions would see
a homogeneous universe at later times.
An even more exotic possibility is that the bounce squeezes the universe so
tightly that all the black holes merge. Even the supermassive black holes we
know exist today could lead to this situation, if our universe eventually
recollapses. These progressive mergers would generate black holes with a
hierarchy of increasing mass until, eventually, the whole universe would be
turned into a black hole.
Nobody knows what would happen in this situation, but work by two groups has
recently thrown light on the problem. Last year, Daniela Pérez and Gustavo
Romero at the Argentine Institute of Radio Astronomy and, independently, a
team led by Maxence Corman at the Perimeter Institute in Canada, calculated
the behaviour of a single black hole during a bounce. Although the details
of their calculations differ, both groups agree the black hole could survive
through the bounce and that its size may shrink for some period. This
shrinking also raises the possibility that black holes may never completely
merge.
All of which is well and good, but what about finding evidence?
Interestingly, another recent study offers some hope that we might one day
be able to identify pre-big bang black holes, meaning that we could
distinguish them from black holes formed in our universe. It was led by
Yi-Fu Cai at the University of Science and Technology of China, who is
interested in the idea that primordial black holes might have generated the
supermassive black holes at the centres of galaxies.
Supermassive black holes
These huge black holes range from 1 million to 10 billion times the mass of
the sun. We know from looking at the distant universe that they already
existed very early on – possibly too soon for them to have been created by
standard astrophysical processes. It isn’t clear how they could grow so big,
so fast. One possibility, although not the mainstream view, is that they
were seeded by primordial black holes. In which case, is there some way to
figure out if these primordial black holes came from a big bang or a big
bounce?
Cai and his colleagues modelled the density fluctuations in the inflationary
and bounce scenarios, to compare the two models. They predict that the
number of supermassive black holes would fall off more steeply with
increasing mass in the case of a bounce. At the moment, we don’t have enough
data to discriminate between the two scenarios. But future observations by
the James Webb Space Telescope could provide this.
The existence of primordial black holes formed in this universe is
speculative, so the notion of black holes from a previous universe might
seem doubly speculative. Nevertheless, it is important to explore this
possibility, not to mention exhilarating. Just as thinking about primordial
black holes has led to important insights into quantum gravity, thinking
about pre-big bang black holes may lead to further physical insights, even
if it turns out that the universe isn’t cyclic.
I have recently retired, and I find it strangely appropriate that my career,
which began with the study of black hole formation at the start of this
universe, is finishing with the study of their formation at the end of the
last one. My article 50 years ago concluded that “black holes are as
pervasive in theory as they are evasive in observation”, but I am now more
optimistic about finding primordial black holes, whether or not they formed
in a previous universe.
Source: New Scientist
Author (Bernard Carr) of this article is emeritus professor of mathematics
and astronomy at Queen Mary University of London
Tags:
Space & Astrophysics