Universes are There?
Response to a Quora Question
to my Website
The Question—Stephen Hawking said in his last post before he died that the
number of universes was finite. Why would he do such a thing? Does the fact
that it is finite mean that no new universes will ever form again?
below first appeared on Quora. Their
policy, which I presume is an agreement, is that they retain non-exclusive
rights to answers by members. Changes made here but not made in the Quora
answer are marked in differently
My view is that current physics
warrants no conclusion at all as to the number of ‘universes’.
Before explaining why I think so, I
will clarify the meaning of the term ‘universe’. If ‘universe’ means all that
there is over all time and space then there is and can be only one universe.
I will use the term ‘universe’ in just that sense. So what does it mean to talk
of the plural, ‘universes’. In that use we are speaking of something rather
like our empirical cosmos. So where the question uses the term ‘universe’ it
refers to ‘cosmoses’. We could use a term like sub-universe, mini-verse, but
I will use the term cosmos. I avoid the term multiverse altogether.
I should tell you a little about my
background too. Though I’m an engineer, my interests are ‘omnivorous’. In
physics, as a grad student in engineering, I took most of the graduate (PhD)
level theoretical courses in physics—classical mechanics, quantum mechanics,
electromagnetism with special relativity, and general relativity—in which I
performed better than most (PhD) physics students. However, I have not used
that experience professionally or to do research. I’m still keen on
theoretical physics but I do not think of myself as an expert.
I will now explain my view.
At the end of this answer there are two
quotes that show that Stephen Hawking held that theoretical models of the
empirical universe are true of the entire universe—seen or empirical as well
To hold that that is true one must
assume that the entire universe is (a) like our empirical cosmos with regard
to our physical laws and (b) is also described by models that describe our
cosmos at least down to just after the big bang singularity.
While, as far as physics goes, it is
speculative to talk of other cosmoses, because our models are only and
strictly of the empirical, it is also speculative to say that there is no
For example, even if there is no time
before the big bang on our models, it does not follow that there was no time
at all. If our cosmos is finite on our models, it does not follow that the
universe is finite. If the cosmos is infinite on our models, it still does not
mean that the models capture or that the cosmos is the entire universe—for
the universe may be even larger in magnitude and or dimension and the cosmos
embedded in it. In that case, the cosmos may be almost observationally and
structurally isolated from the rest of the universe (at present and since the
big bang). Of course if the isolation is absolute then it makes no sense to
talk of a larger or embedding universe but it is only in the models that the
isolation is known to be absolute.
So the claim that there is only this
universe is the result of (i) placing unwarranted faith in our models of the
empirical cosmos and (ii) lack of imagination.
Some physicists and others assert that
it is speculative metaphysics to claim or even talk of other cosmoses. On the
basis of current physics, that is true (but not unwarranted if one does not
pretend that it is more than speculation). On the other hand the claim that
there is but one cosmos or just a few is just as speculative. It results from
taking our theoretical models too seriously (and perhaps even personal
investment in the models). This, by the way, has happened before in physics,
e.g. toward the end of the nineteenth century just before the relativity and
The correct view from physics,
according to the argument above, is that the extent of the ‘greater
universe’ and the number of sub-universes is unknown.
Here are the quotes. It’s not necessary
to read them to understand my argument above. Both quotes are rather long but
not essential to my argument (there is more in the linked articles). Neither
article is technical.
The first is from (a
lecture by Hawking)—
of our past light cone implied that time must have a beginning, if the
General Theory of relativity is correct. But one might raise the question, of
whether General Relativity really is correct. It certainly agrees with all
the observational tests that have been carried out. However these test
General Relativity, only over fairly large distances. We know that General
Relativity can not be quite correct on very small distances, because it is a
classical theory. This means, it doesn't take into account, the Uncertainty
Principle of Quantum Mechanics, which says that an object can not have both a
well defined position, and a well defined speed: the more accurately one
measures the position, the less accurately one can measure the speed, and
vice versa. Therefore, to understand the very high-density stage, when the
universe was very small, one needs a quantum theory of gravity, which will
combine General Relativity with the Uncertainty Principle.
hoped that quantum effects, would somehow smooth out the singularity of
infinite density, and allow the universe to bounce, and continue back to a
previous contracting phase. This would be rather like the earlier
idea of galaxies missing each other, but the bounce would occur at a much
higher density. However, I think that this is not what happens: quantum
effects do not remove the singularity, and allow time to be continued back
indefinitely. But it seems that quantum effects can remove the most
objectionable feature, of singularities in classical General Relativity. This
is that the classical theory, does not enable one to calculate what would
come out of a singularity, because all the Laws of Physics would break down
there. This would mean that science could not predict how the universe would
have begun. Instead, one would have to appeal to an agency outside the
universe. This may be why many religious leaders, were ready to accept the
Big Bang, and the singularity theorems.
It seems that
Quantum theory, on the other hand, can predict how the universe will begin.
Quantum theory introduces a new idea, that of imaginary time. Imaginary time
may sound like science fiction, and it has been brought into Doctor Who. But
nevertheless, it is a genuine scientific concept. One can picture it in the
following way. One can think of ordinary, real, time as a horizontal line. On
the left, one has the past, and on the right, the future. But there's another
kind of time in the vertical direction. This is called imaginary time,
because it is not the kind of time we normally experience. But in a sense, it
is just as real, as what we call real time.
directions in space, and the one direction of imaginary time, make up what is
called a Euclidean space-time. I don't think anyone can picture a four
dimensional curve space. But it is not too difficult to visualise a two
dimensional surface, like a saddle, or the surface of a football.
In fact, James Hartle
of the University of California Santa Barbara, and I have proposed that space
and imaginary time together, are indeed finite in extent, but without
boundary. They would be like the surface of the Earth, but with two more
dimensions. The surface of the Earth is finite in extent, but it doesn't have
any boundaries or edges. I have been round the world, and I didn't fall off.
The second quote is from ( was
a “key collaborator” of Stephen Hawking)—
of the big bang predict that our local universe came into existence with a
brief burst of inflation – in other words, a tiny fraction of a second after
the big bang itself, the universe expanded at an exponential rate. It is
widely believed, however, that once inflation starts, there are regions where
it never stops. It is thought that quantum effects can keep inflation going
forever in some regions of the universe so that globally, inflation is
eternal. The observable part of our universe would then be just a hospitable
pocket universe, a region in which inflation has ended and stars and galaxies
theory of eternal inflation predicts that globally our universe is like an
infinite fractal, with a mosaic of different pocket universes, separated by
an inflating ocean,” said Hawking in an interview last autumn. “The local
laws of physics and chemistry can differ from one pocket universe to another,
which together would form a multiverse. But I have never been a fan of the
multiverse. If the scale of different universes in the multiverse is large or
infinite the theory can’t be tested. ”
In their new
paper, Hawking and Hertog say this account of eternal inflation as a theory
of the big bang is wrong. “The problem with the usual account of eternal
inflation is that it assumes an existing background universe that evolves
according to Einstein’s theory of general relativity and treats the quantum
effects as small fluctuations around this,” said Hertog. “However, the
dynamics of eternal inflation wipes out the separation between classical and
quantum physics. As a consequence, Einstein’s theory breaks down in eternal
that our universe, on the largest scales, is reasonably smooth and globally
finite. So it is not a fractal structure,” said Hawking.
The theory of
eternal inflation that Hawking and Hertog put forward is based on string
theory: a branch of theoretical physics that attempts to reconcile gravity
and general relativity with quantum physics, in part by describing the
fundamental constituents of the universe as tiny vibrating strings. Their
approach uses the string theory concept of holography, which postulates that
the universe is a large and complex hologram: physical reality in certain 3D
spaces can be mathematically reduced to 2D projections on a surface.
Hawking and Hertog
developed a variation of this concept of holography to project out the time
dimension in eternal inflation. This enabled them to describe eternal
inflation without having to rely on Einstein’ theory. In the new theory,
eternal inflation is reduced to a timeless state defined on a spatial surface
at the beginning of time.
“When we trace
the evolution of our universe backwards in time, at some point we arrive at
the threshold of eternal inflation, where our familiar notion of time ceases
to have any meaning,” said Hertog.
earlier ‘no boundary theory’ predicted that if you go back in time to the
beginning of the universe, the universe shrinks and closes off like a sphere,
but this new theory represents a step away from the earlier work. “Now we’re
saying that there is a boundary in our past,” said Hertog.
Hawking used their new theory to derive more reliable predictions about the
global structure of the universe. They predicted the universe that emerges
from eternal inflation on the past boundary is finite and far simpler than
the infinite fractal structure predicted by the old theory of eternal
if confirmed by further work, would have far-reaching implications for the
multiverse paradigm. “We are not down to a single, unique universe, but our
findings imply a significant reduction of the multiverse, to a much smaller
range of possible universes,” said Hawking.