So we want to go on to a little bit of science and tonight I've chosen to focus on a couple
of things in cosmology. So I'm not a cosmologist, that's not my personal area of deep expertise
but it's where some of the really interesting SKA science lies. So I'll talk briefly about
two things. One is probing the dark ages and the second thing will be something about
dark energy which I think is possibly the most exciting thing that the SKA can tackle.
So just before we do that I have to go through a little bit of physics with you just as background.
First bit of physics is this. So hydrogen, the simplest element, one proton, one electron
in its simplest form. There's a lot of it in the universe. The universe started out
as overwhelmingly hydrogen with just a little dash of helium thrown in. So after the big
bang cooled off it was hydrogen that formed and everything else formed from hydrogen.
Most of the helium, carbon, nitrogen and oxygen go through the entire periodic table comes
after and all of those elements were created when balls of hydrogen collapsed into stars,
hydrogen burned into helium, helium burned into carbon etc etc. So everything that you
and I made of, everything that you can see around you is the product of that stellar
nucleosynthesis process, converting hydrogen into heavier elements. But even over the course
of the universe all of that conversion of hydrogen into other things is barely made
a dint in the hydrogen. So the universe is mostly made of hydrogen. So it's a very, very
fundamental element for astronomers. It has a really, really beautiful property without
which radio astronomy would be in deep trouble. So I've got a hydrogen atom here, proton,
electron and these particles have a property called spin which is a quantum, a quantum
mechanical property. So it turns out that if you have hydrogen atom where the spins
of the proton and the electron are aligned every once in a while, a few times within
the age of the universe, those spins will just spontaneously flip a quantum mechanical
process, completely random, spontaneous and it flips into this configuration where the
spins are anti-parallel. And it turns out that this is a lower energy state than this.
So the excess energy has to leave the system and the excess energy leaves the system as
a radio photon with a very, very, very specific frequency. So it produces radiation at 1.420406
approximately gigahertz. That's around about a wavelength of about 21 centimeters, very,
very famous radio emission in radio astronomy.
So if we tune our radio telescopes to 1.42 gigahertz, we can directly observe hydrogen
and here's a map of the hydrogen gas in our galaxy. So again, here's the plane of the
galaxy where most of the hydrogen sits. So keep that in mind that this is a very, very
useful property of hydrogen for radio astronomers.
So I also need to introduce a concept called redshift. This is something that you are all
familiar with. If you've been out on the streets and you've had a police car or an ambulance
screen past you with its siren on, you'll notice that when you're in front of the car,
you hear a very high pitched siren. As soon as the car passes you and you're behind the
car, the pitch drops dramatically. This is because when you're in front of the car, the
car is actually catching up to the waves that the siren emits and so the waves get bunched
up and you get a high frequency sound, high pitch. The opposite occurs in the opposite
direction. The car is racing away from the waves it emits and so the wavelength is much
longer and that gives a low frequency and a lower pitch. This is an effect called the
Doppler shift. So it's exactly the same in astrophysics except we can use the signals
from the radio signals from hydrogen gas instead of a siren and a galaxy instead of the car
and we stay where we are on Earth. So because the universe is expanding, galaxies at different
distances are moving with different speeds and the emission that comes from these galaxies
when it's received back at the Earth at our telescope is going to have a different frequency.
So if we're looking at a galaxy that's right next door to our Milky Way galaxy relative
speed of zero kilometers per second, we point a radio telescope at it, we see hydrogen at
1.4 gigahertz. If we look at a more distant galaxy, say this one that has a speed of 100,000
kilometers per second relative to us due to the expansion of the universe, we point our
telescope at it, look at hydrogen and we find that it pops up at 1 gigahertz, not 1.4 and
so on and so forth. At a faster speed, a bigger distance, the frequency received on the Earth
drops. So this is the fundamental observation that Edwin Hubble made back in the 1920s when
he looked at galaxies, realized that the light from the galaxies was redshifted and said,
aha, the universe is expanding. So this is direct evidence of an expanding universe.
And most of cosmology, cosmology being the detailed understanding of the structure of
the universe, the spacetime geometry of the universe, most of cosmology boils down to
understanding the relationship between distance and redshift. So the different cosmological
models predict different things about the relationship between distance and redshift.
And telescopes like the SKA will make observations that test those models.
So first, so with that little bit of physics background, I'll first talk about looking
at the dark ages of the universe. So here again is a schematic. We have the Big Bang.
For a tiny fraction of a second, the universe expanded very, very quickly, a period known
as inflation. During that time, the universe was completely composed of fully ionized atoms,
so basically free protons and free electrons, and also radiation. All mixed up together,
incredibly hot soup of material, so hot and so dense that if a photon or some radiation
propagated through the universe, it could only move a very small distance before colliding
with a particle being scattered, shot off in another direction, reabsorbed. So it was
this massive mess. After about 300,000 years, the universe had
cooled and expanded to a point where the electrons and the protons could combine to form hydrogen,
which is a neutral material. As soon as that happened, the radiation was then free to propagate
for very, very long distances, so it was no longer constrained as part of this fully ionized
soup. So as soon as the radiation started propagating,
it could travel through the rest of the universe, and after 13.7 billion years, it can reach
us to be looked at with a radio telescope. So this is what you get when you look at this
radiation that's left over from this period in the universe. So this is the cosmic microwave
background, a map made with a radio telescope in Earth orbit, and these little fluctuations
are very, very fine density perturbations left over from that period. When the universe
was this soup of material, there were all sorts of sloshing back and forth of the material,
sort of like waves. They're called acoustic oscillations. You can think of them as waves
on the surface of a lake, and the material sloshed around like mad, but as soon as the
material became neutral and the radiation couldn't drive that sloshing, the last moment
of those waves was frozen in, and so this is the frozen in imprint of the last state
of that oscillation. And it's the imprint that all of the structure formation in the
universe followed after that. So these tiny little density fluctuations grew into clusters
of galaxies and superclusters of galaxies. But all of that neutral material was left
behind, all of that hydrogen was left with this imprint and took some time to start forming
stars and start forming galaxies, had to collapse gravitationally, take some time. After some
time, the stars and galaxies formed and started radiating their own light at radio wavelengths
and optical wavelengths. And by the time you get to the present day, you're sort of surrounded
by galaxies. So you can use all sorts of telescopes to look back a long way into the universe,
but with an optical telescope, you can never look back to that time in the early universe
before the stars and galaxies are actually formed because they're emitting no visible
light. But it's all hydrogen, right, which is very convenient for radio astronomers because
we can detect hydrogen because it has this spin flip transition that allows us to look
at it. So with the SKA, it will be big enough to directly observe that hydrogen gas millions
to a billion years after the Big Bang in its phase of forming the first generation of stars
and galaxies. And it will be the only instrument capable of doing this. Remember the redshift?
So by the time the 1.4 gigahertz radiation gets to the Earth, it's redshifted down to
100 megahertz. So the SKA goes down to 100 megahertz and it has enough collecting area,
enough sensitivity that it can see back to that period in the universe. Right, this is
where I need to take a deep breath and wrap a cold towel around my head because dark energy,
which is a bit of a mystery. So it starts with a bit of a history lesson. In 1917, everyone
thought that the universe was static. That is, that the galaxies were fixed, nothing
really changed. This worried Einstein because his formulation of the equations of general
relativity predicted that in that state, the universe should just collapse. So how did
we all manage to be here in a universe that's not collapsed? And he introduced into his
equations an ad hoc element called a cosmological constant, which had the effect of maintaining
a static universe against gravity, sort of an anti-gravity constant. Around about 12
years later, Edwin Hubble was releasing the results of his studies over the last few years
in which he directly obtained evidence that in fact all of the galaxies were rushing apart
from each other. The universe is not static at all. At which point Einstein called the
cosmological constant his greatest blunder because it wasn't needed, never was needed.
It was just simply that they lacked the observational understanding of the universe. So he threw
it away and felt quite bad about it. However, some 70 years later, in the late 1990s, some
really startling observations were made of supernovae, exploding stars in other galaxies
that you can see to very large distances, therefore very high redshifts. And it's alleged
that every supernova has the same brightness because it's the same underlying physical
principle that causes the star to explode and releases the same amount of energy each
time. So if you look at these things at different distances, they should follow a well-known
case in dimness. So they should get fainter according to a certain relationship. But what
these guys found was that that relationship didn't hold. And they surmised that the universe
is actually expanding faster and faster as the universe gets older and older due to some
unseen additional force that's not understood by physics, and they called it dark energy.
So this looks very much like a cosmological constant. And so people were saying, well,
perhaps Einstein was right all along, and we actually do need this cosmological constant.
So just last week, actually, there were some new results released from a team at Swinburne
University that basically confirms that dark energy exists and suggests strongly that it
can be explained in terms of Einstein's equations for gravity with a cosmological constant added.
And the bottom line is where we want to be at in sort of 15 years time. That is the SKA
measuring this in detail, looking at dark energy back to sort of 9.5 billion years in
the universe. So, OK, why is that interesting? Who cares? Dark energy, so what? So there
were two explanations for these observations. Explanation one was just that Einstein was
wrong and his theories needed modification. The second explanation, which looks to be the
right one, is that Einstein was actually right, but there's a new and unseen particle or field
that pervades space and is a force that counters gravity and pushes the galaxies further and
further apart faster and faster as the universe evolves. So that can take the form of the cosmological
constant in Einstein's equations and equates to what we call a non-zero vacuum energy.
So in quantum mechanics, all the time around you in every volume of space, particles are
being created and destroyed and we never see them because they're created, they're destroyed
virtually instantaneously. This is happening all around us. The theorists tell us. But the
key is that whatever's created is always destroyed, so it's a net energy gain zero. So if you
had slightly more things created than were destroyed, you'd have a non-zero energy input
into the vacuum and that would be something that would push space apart and create something
that looks like this dark energy. So that's a really massive revision of the laws of physics
if such a thing were possible because for the first time we have observational evidence
of something that connects gravity with subatomic physical processes, that is quantum mechanics.
So the unification of gravity and quantum mechanics has been the holy grail of physics
for the last 50 years and dark energy looks like it could be the key to understanding
how gravity and quantum mechanics are related to each other. So we know that dark energy
exists, we've got no idea where it comes from, could be the vacuum of space, could be a non-zero
vacuum energy, how is it created, how is it related to quantum mechanics, how does it
evolve over the history of the universe? So we know this stuff exists and in fact we have
a pretty good idea that dark energy makes up 74% of the mass energy content of the universe,
another 20 odd percent in something that's called dark matter which is also very, very
poorly understood and this tiniest liver up the top is you and I. That's the atomic
material, atoms, molecules, proteins, wood, steel, flesh, everything that you ever see
around you and are ever going to see is only 4% of the universe. So 96% of the universe
we really don't understand, in particular the 74% dark energy. So the SKA is going to
search for this dark energy, the Large Hadron Collider is going to search for dark energy,
so the SKA is taking an astronomical scale approach by looking for the effects of dark
energy over cosmological distances, the LHC is going to search for it by smashing particles
together really hard and seeing what comes out. So we're so looking in the face of the
first really major revision of the laws of physics for the last 70 or 80 years. So the
last revolution in physics was our understanding of quantum mechanics mainly through the 1920s
and 1930s. And many people say yes, well this is all very nice, it's all highly esoteric,
so what? Who cares about quantum mechanics? Well, I'd simply say that every single electronic
device that we use, the tens or hundreds of trillions of dollars in the electronics industry
and the productivity gains that we have by using computers and all manner of modern devices
are only possible because we understand quantum mechanics. So that's a tens of trillions
of dollars a year industry that didn't exist 100 years ago and only exists because we understand
the fundamental laws of physics, in particular quantum mechanics in that case. And this has
always happened over history. You make a fundamental advance in science and everyone
thinks it's esoteric, but sooner or later, maybe after decades, clever people figure
out what this actually means, how to build new and better technology. And that's what's
happened with quantum mechanics and other sciences. So unifying gravity and quantum
mechanics might sound esoteric, but whenever you're revising the laws of physics, you're
not quite sure what that's going to bring in 50 years time or 100 years time. So these
are the really fundamental advances that are at the absolute core of technology. So some
people like that, some people like me just like to understand how the universe works.
So how is the SKA going to do this? So I might spend just a minute or two explaining.
So you'll recall that I said that cosmology boils down to finding out the relationship
between redshift and distance. So this curve is the prediction of Einstein's equations
with a cosmological constant included. And currently we have three observational data
points. This was released last week and is the result of five years work by a team of
roughly 20 people who looked at a quarter of a million galaxies. So all of that work
boiled down to one data point on a graph. Happily it agrees with the theoretical curve.
So indications are that Einstein's theory of gravity is correct and we need something
like a vacuum energy in the form of a cosmological constant to explain the observations. It's
quite interesting how you go about this observation. So here we have the microwave background again.
And from this you can figure out that things in the universe have a preferential clustering
scale. So they have a small preference for being at a certain distance from each other
compared to other distances. And you can basically measure that. That's measured in the imprint
of the cosmic microwave background. And then that distance stays constant over the evolution
of the universe. So it's like having a standard ruler. So if you've got a 30 centimeter ruler
and you put it at different distances it appears to have a different size. So you can measure
the angular extent of that ruler. You know how long it is. So simple geometry you can
figure out its distance. So you do this by looking at galaxies over the history of the
universe. And so you measure the angular size of this ruler and you measure the red shift
of the galaxies that form that ruler. From that you can form the red shift distance relationship.
And that's how these three observational data points were formed. So this was made with
about a quarter of a million galaxies. The SKA will do this properly with a billion galaxies.
So you won't be just getting one data point on this curve. You'll be getting high precision
stuff. This cuts off at about 6 billion light years back in time. SKA will go out to something
like 10 billion light years. So you get a very high precision measurement and hopefully
understand a lot more about the nature of the dark energy.
