So now we have to face up to
the distinction between observing how
the Universe behaves, and
figuring out if there's a unique
physical explanation for it.
At first sight, it seems like we've
destroyed ordinary physics, because
surely having an energy density associated
with empty space, makes no sense.
Maybe, it's not so surprising though.
And the physics that one reaches for
at the first instance, is the quantum
mechanical uncertainty principle.
What this says is that perfect
knowledge of the position and
speed of a subatomic
particle is impossible.
So effectively, nothing
can be completely at rest.
Otherwise you'd know exactly
where it is, and that it wasn't moving.
This is important,
if we think about some of the constituents
of what seems to be empty space.
One of which is electromagnetic radiation.
Now you're seeing me because of
electromagnetic radiation. What is it?
Well,
what we say is that the space between us,
is filled with electromagnetic field.
Now, nobody actually knows
what the electromagnetic field is,
but it's a thing whose existence
we see in the universe.
And you're familiar with it probably
through the phenomena of lines of force.
Probably everybody at school sprinkled
iron filings around a magnet and
watched the patterns they generated.
So, empty space has electromagnetic
lines of force in them.
And radiation corresponds to waggling
those elastic strings as it were.
Now, the Uncertainty Principle, though,
says we can't have zero oscillation
energy in those springs, because that
would be like a state of perfect rest.
So, in fact, here must be some
zero-point energy, as it's called.
And just as a consequence of that,
it's obvious
that it'd be, it would be extremely
surprising if the vacuum contained no
energy at all.
Because it would be inconsistent with
what we know about quantum mechanics.
So let me now try and convince you
that if empty space can have density,
it can also have repulsive
gravitational properties.
This is the same argument
as with expansion of the universe.
If something moves, kinetic energy tends
to reduce as gravity opposes the motion.
So a small sphere of matter in expansion,
as it grows larger,
will move less rapidly,
because its gravitational energy
has changed, and it drains away the,
the kinetic energy.
But, if I have a sphere,
a vacuum, that expands,
then, because the vacuum has
a certain amount of mass
per unit volume, as the volume is bigger,
the mass inside is correspondingly larger.
And that means that
the gravitational effects,
rather than becoming smaller as the
sphere, gets larger, become even bigger.
And the only way to conserve energy is for
the kinetic energy to rise in addition.
So here, M is fixed for ordinary matter.
Here, the mass rises in
proportion to the volume for the vacuum.
And it's this
that gives us the tendency to,
for the expansion of
the universe to accelerate.
So now a picture will be, that's the size
of the universe versus time, emerges
with a big bang, tends to decelerate,
but then, goes through an inflection.
Because at late times,
the vacuum energy dominates.
And we seem to live more or
less where this inflection is happening,
right at the present.
So what I have given you so
far, is more or
less the standard consensus
approach to cosmology.
But some skepticism is clearly in order.
Because the conclusions
that we reach about the existence
of dark matter,
about the existence of dark energy,
clearly depend on
the physics that we assume.
So it's possible that the,
the laws of physics that
have been assumed,
are simply incorrect.
After all,
the law of gravitation for example,
is established by observations on earth or
within the solar system.
Isn't it a huge extrapolation to
think it might apply to the whole
universe without modification?
Well, other possibilities
have been suggested.
For example, in terms of getting rid of
dark matter, you could consider MOND.
Stands for Modified Newtonian Dynamics.
What is Newtonian Dynamics?
It says F equals M times A.
So the force from gravity,
dictates the acceleration of particles,
how rapidly they change their speed.
But if you think about
galaxy rotation curves,
that's at large distances from
the center of the galaxy,
the accelerations are much lower than
we've ever measured in the laboratory.
So, MOND suggests that maybe this
fails for the low accelerations.
That theory could actually
account perfectly well for
the rotation curves of galaxies.
Similarly, we can get rid of dark energy,
which we reached as a conclusion
by adopting the Friedmann Equation,
which comes from Einstein's gravity.
So, if we simply said that
Einstein's gravity was
not the correct theory
dark energy might not exist
as a physical substance.
It could be simply an optical illusion.
So how do we decide, which we believe?
Do we take conventional
physics with dark matter and
dark energy, or
do we modify the physics?
Are these approaches
completely equivalent?
Not necessarily.
One thing you can do is you can look for
consistency.
That is, you can ask whether
a changed theory could actually
explain other things that
the old theory couldn't explain.
So for example,
in the case of dark energy we have not
only the size of the universe as
the function of time to explain.
But we also have the growth under gravity,
the collapse under gravity
of the patterns in the three-dimensional
distribution of galaxies.
So this sort of thing can be measured, and
you could ask whether it occurs at the
correct rate for standard Einstein gravity.
Which it does at the level of
precision we can probe so far.
So, this tends to favour
a conventional approach to
gravity as opposed to modifying
Einstein's theory, for example.
But there's an alternative way of,
of thinking about this which is
less to do with direct tests,
because obviously there can only be
a finite number of tests of a theory.
It has to do with prejudice,
and in science there's
a well defined probabilistic framework for
trying to assess credibility of theories,
which goes under the heading of
Bayes' theorem in statistics.
Now what this does is it
turns a degree of belief,
which should be a personal quantity,
into a probability, which
can be quantified, and subject to the
usual rules of combining probabilities.
What Bayes' theorem says, is that
the thing that we would like to know,
which is the probability
of a theory being true,
given the existence of some data,
is proportional
to the probability of
getting that set of data,
given that the theory is true,
times a prior probability,
a degree of plausibility, if you'd like.
From this point of view, one can see why
it is that the scientific community is
reluctant to adopt radical new
modes of explanation like MOND.
It might do a perfectly good job of
accounting for the observations.
If we assume MOND we can certainly
account for galaxy rotation curves.
But MOND is a theory that was
created purely to explain one
particular astronomical observation.
Whereas standard gravity, of course,
grows out of a whole range of observations
here on Earth or in the solar system.
So, therefore,
the prior probability of MOND,
its general plausibility,
seems much lower to most physicists.
And this is why the burden of proof for
new theories is very high.
And it seems to be correct, because we
have to account for the fact that one
brings prejudices, or experience with
efficiency of physics in other contexts.
And that has to be included into such
a framework before a paradigm shift will
actually happen,
and a new theory be considered to be more
probable in the context of the data.
In concluding this quick run down
of the modern picture of cosmology,
with dark matter and dark energy,
you might wonder if any of these
ideas have any lasting value.
After all, societies throughout history
have had their ideas about the nature of
the universe.
Is this just another passing creation
myth that will be laughed at
in a thousand years?
In the same way as we
pour scorn on those who
thought that the world was a plate
supported on the back of a giant turtle.
I would argue not.
It's not that we're any
cleverer than the Ancients, but
we're fortunate to
benefit from technology;
telescopes large enough to
show us a significant fraction
of the entire visible universe.
The maps that we've made won't change.
They have high enough fidelity now that we
can see the universe the way it really is.
And people have been waiting for this
technology to arrive throughout history.
We're lucky enough to be the ones
here on the spot to use it.
