inflation is not the Big Bang inflation
is not the Big Bang no the Big Bang is
inflation if you like gives the initial
conditions for the Big Bang
well the Big Bang by itself is that
initial singularity which we don't
understand no one can go back to the
very beginning and anyone who tells you
they can is there's a little bit of
mystique going on in to say the least so
what we normally think of as the Big
Bang Europe is you're thinking of just a
few nanoseconds after the actual initial
singularity the Big Bang has certain
assumptions in it I'm going to come to
inflation why you need inflation and
what it is the Big Bang has certain
assumptions in it is based on what was
called the cosmological principle which
is something I'm Stein introduced in
solving his equations the cosmological
principle states that the universe on
large scales is homogeneous that means
we're in no unique place in the universe
we're just representative of the whole
universe and it's isotropic that means
it looks the same in all directions once
you've got those initial that initial
principle in place and once you've got
something that's expanding out with the
particles are expanding out then you can
begin to work out what the temperatures
of the universe how rapidly the universe
should expand what it should look like
today and you can make all sorts of
predictions and they worked really well
it gives you concepts it gives you time
scales like when does nuclear synthesis
occur when did the first nucleus occur
after about three minutes it tells you
when the microwave background radiation
emerged about 300,000 years after the
Big Bang it tells you when radiation
matter had the same density and when we
moved into a matter-dominated universe
and structures would begin to form and
so it gives you all of these nice
results it does not tell you where did
structures come from in the first place
what provided the initial seeds for
structures it does not tell you why in
the very early universe it should be so
homogeneous and isotropic why did that
happen and it does not tell you how to
get rid of objects that could have
formed in phase transitions in the early
universe in
objects called monopoles which could
form in grand unified theories and if
you can't get rid of these mana poles
they're so massive that they would have
caused the universe to collapse very
quickly so there are these issues which
the Big Bang has associated with it that
need explaining earlier it was to try
and understand why the universe was as
it is so flat is especially incredibly
flat homogeneous and isotropic that led
to people like in particular Alan Guth
from MIT and then Soviet physicists like
Alexis Terra bin ski and Andrei Linde
came up with these ideas to provide you
if you like with the initial proceeding
dynamics which would then feed into what
we now call the Big Bang and that was
called inflation inflation and here we
go an inflation corresponds to a period
where the universe actually accelerates
the size of the universe actually grows
exponentially quickly and so it and the
reason it and it's a very short period
of time in the early universe this
occurred for about you know about 10 to
the minus 30 seconds that's all you
require in order to satisfy all these
conditions it could have spent much much
longer but the amount you we require to
explain the observed universe is about
this amount it's it's it's not very long
at all you have an initial very small
patch of the universe that was
represented by the gap there okay and
that initial small patch is such that
all the radiation that's present in
there has had time to interact with each
other it's in what's known as causal
contact so light has had a chance to
propagate from one side to the other
side that in other words you expect this
bit of the universe to be at the same
temperature now this piece of the
universe just then begins to grow really
rapidly
it just gets stretched by the expansion
the expansion stretches it exponentially
rapidly for this short period of time
but it grows so rapidly that within it
it's now easy to have a piece of
which becomes our observable universe
within this huge huge humongous piece
it's now easy to have as much smaller
patch which becomes our observable
universe and because it's begun from
this region which was causally connected
and is nice and smooth and homogeneous
it emerges has this smooth homogeneous
isotropic region which is what we now
see and so you have an explanation of
how you can go from the universe which
is on huge scales smooth and hope and
isotropic and homogeneous from a region
which was much smaller earlier on just
blowing up a photo if you take a photo
of your family and blow it up and put
our cameras on the wall it's going to be
the same picture it's just bigger it's
it's it's much bigger yeah and but the
difference is that the that the universe
that you might correspond to might just
be the ear of one of those of that of
that photograph it doesn't have to be
the whole thing anymore I should explain
when when we look at the uniform nature
of the this microwave background right
it's it's got this beautiful smooth
temperature it's about 2.75 Kelvin today
and it was much hotter earlier on but
it's 2.75 and it's incredibly small
deviations on either side of it a few
parts in 100,000 so it's almost uniform
throughout and that's our whole
observable universe okay now this
radiation was was released about 300,000
years after the Big Bang it's when the
Cosmic Microwave Background was released
when the when the the first electrons
began to bind with it with the protons
to form and here hydrogen atoms if you
look at the size of the universe at that
point 300,000 years after the Big Bang
and say right let me compare that size
of the universe to what we see now it
turns out that there is simply no way
that one region of the sky that we can
see today which has this temperature of
2.75 degrees Kelvin could have
communicated with this region of the sky
which also has a temperature of 2.75
degrees coming
you know each of these patches couldn't
have grown took to meet each other and
so this is again another motivation for
having the idea of inflation those two
patches which today look completely
distinct from wandering actually came
from the same initial small connected
patch which then grew exponentially
rapidly this seems like what we all know
that we all know that everything was
really really small and then expanded
yeah we know it's like a balloon no you
see the the the standard cosmology it
does have things expanding but not at
the rate at which you require so if you
just solve your equations for a universe
full of radiation or full of matter it
will expand you're quite right and but
when you work out the rate at which it
will expand it's not enough to then
account for these this observation of
the uniformity of the Cosmic Microwave
Background so this isn't like a car
rolling down a hill
someone has pressed an accelerator yes
yes someone's pressed that accelerator
and so something has got to provide that
initial energy energy density energy per
unit volume in order to cause the
universe to do this and this is where we
once again and introduce the
cosmologists for particle cosmologies
favorite friend which is a scale of
field and in up until March of this year
we used to say scalar field but no one's
ever seen a fundamental scalar field of
course we've now got the Higgs it's got
Nobel Prize associated with it so we can
all proudly talk about scalar fields and
this is another example it's a different
type of scalar field it's not a not
necessarily a Higgs field it's a but
it's a it's an object that's just got a
value and magnitude and it has a
potential energy associated with it and
in the early universe this potential
energy dominates over everything else
all the energy and the radiation and any
matter that's there it's much less than
the potential energy of this scalar
field and that is enough to drive this
period of
of acceleration and then eventually as
the universe is expanding Excel you know
growing exponentially quickly this field
it doesn't stay on the top of this hill
you can imagine the field as if it's on
it on it on a very on a hill there it's
got a very flat region and then it drops
down right it's going down to its minima
the field is gradually rolling down this
hill and as it's up at the top here it's
it's fairly stationary this energy is
constant it's this potential energy is
constant and then eventually it begins
to experience the steeper part of the
hill and the field quickens up and at
that point as it quickens up this
natural period of inflation this
acceleration stops and you you leave
that accelerating period the universe
then has to go into what's called the
reheating all of that energy is stored
in this field has to get transferred
over into the particles that you and I
are made of because that they've all
been diluted away during this period of
expect exponential expansion anything in
the universe has just been diluted away
as the universe expands so rapidly so
we've got to re re energize the universe
we've got to create the particles again
and this process is called reheating and
it's a very difficult process to
actually understand because it's in the
sense of the field has to couple to lots
of other particles and we have to
understand how they're created and
that's an ongoing area of research
trying to understand how efficiently
this is done but the universe reheats it
creates the particles that you and I are
made of and that that then enters a
radiation-dominated era and that if you
like is what we would call the beginning
of the Big Bang so the inflation before
the big in that in the sense I think of
if you think of the Big Bang has been
based on the idea of the cosmological
illogical principle being satisfied and
an expanding universe as its initial
conditions then inflation will give you
that it doesn't inflation still hasn't
addressed this bit about the very
beginning right I've avoided it again
because I don't know the answer to that
I don't know what actually bank to give
me the big to even to spark off that
period
mystery toilet we don't understand with
this you know you can give error to you
yeah yeah
Big Ben one people inflation then
inflation then Big Bang to Big Bang one
is being the initial what happens at
that very easy catch you showed me with
your hand yeah
inflated yeah where did that come from
that was Bob viously band out of the
mystery yeah the creation of space-time
has to create yes so what ever managed
to launch the universe I'll create
space-time wherever that energy initial
energy some people would argue it is
inflation that for example quantum
mechanics allows things allows you to
because of the Heisenberg uncertainty
principle which tells you you can trade
off energy for a fraction of time you
can you can grab a lot of energy for a
short period of time for example in the
Heisenberg uncertainty principle and and
that energy could be the energy stored
in this field and so some people would
say that's what happened that energy was
it was enough to trigger the creation of
space-time and then off it went so in
that sense inflation does it for you
that passions lated whatever was there
like that became diluted hmm what was in
there did it will have had it will have
had radiation yet will have had some
energy in there but not atoms no not a
turn we're too high we're too high an
energy for that there is so if an atom
was to try and thought you know what is
an atom an atom of hydrogen is where an
electron is bound to a proton and and
and proton itself is made up of quarks
okay but if you if you increase the
energy of that electron bound into the
proton if you in fact it takes
relatively little energy to for you to
rip the energy rip the electron off
again and so the early universe had way
way more energy than that so in fact
everything that would be present in the
early universe would have been in their
most basic fundamental form we wouldn't
be able to form structures like protons
even at that stage is it valid for me to
ask how big these things were well how
big was how big was the universe before
and after
inflation so this yeah of course it's
valid so for example a patch and that is
about 10 to the minus 20 I think it's
about 10 to the minus 27 centimeters and
would have grown would have been enough
a big in at work yeah 10 to minus 27
centimeters at about 10 to the minus 30
seconds after the Big Bang would have
been enough to then grow to easily
encompass our observable universe which
at the end our observable universe at
the end of a period of inflation would
be probably about the size of a
grapefruit and then it could carry on
growing after that but it would have
done enough expansion to then account
for the observed o sub P that we see
today
then we went from very small to a
grapefruit through a grapefruit myself
yeah by the end of the period of
inflation that we require there could
have been much more I mean nobody there
are models of the universe because they
require these scalar fields you can
change this potential that's the thing
that you can play around with and get
different predictions there's a very key
thing about inflation which I need to
discuss in a second but different models
will give you different periods of
announce of inflation and but we require
fixed amounts in order to account for
the observed uniformity of this
radiation and the observed flatness
special flatness of the universe but
isn't there also an upper bound to the
inflation could isn't there a point
where you guys will say my life
inflation lasted that long that wouldn't
work I mean have you got it you tell me
this lower bound is this 10 to the minus
13 is there an upper bound
well there's an upper bound no I mean
you can have there are models of what we
call an eternal inflation these are
models where inflation has been going on
in some part of the universe forever
it's just not happening here I mean well
actually it is happening right now with
dark energy but it's at a different
energy scale and so there isn't enough
about
in that sense I mean there are there are
models of inflation where you're it's
constantly popping off different regions
of space growing exponentially quickly
that but they're just not the regions
that we live in it's it's important our
region inflation did end because if
inflation carried on then structures
could never form matter would always be
ripped apart rather than it being
allowed to clump together so we know
inflation how to end it could have gone
on for a long long time in terms of the
amount of doubling of the size of the
universe but we know that we require the
as a fixed amount at the end of the
inflationary period in order to account
for what we see today you talk about a
time when the observable universe was a
sizable break the universe no the
observable universe but the universe
that we saw they you know in flesh and
leads to the size of the universe way
bigger than our observable one and so we
don't know what's going on out in those
regions and it's in those regions that
we could it could still be inflating
there could still be regions of the
universe influencing out there and
spawning more expansion accelerated
expansion there's a good whole group of
people don't particularly like the idea
of inflation and there's a and they
would myth and I've worked on both both
both of these and sides of the argument
and and and one of the reasons is the
question of you know how fine-tuned do
you have to be in order to get inflation
in the very in the early universe and
there is a big question about this using
this argument over this initial patch
for example why should it have been
uniform at this epoch where we expect
quantum fluctuations to be so big that
it could spoil it all for you and so
there is it there is a question about
the fine-tuned aspects of these
inflationary models and that's one way
you you argue about how how how likely
it is that inflation actually occurred
and that's enough that's still a debate
going on as to the
and there are different models in the
others there are models called the
cyclic universe right where the universe
undergoes a series of expansions and
collapses and each each time it
collapses and bounces back
these are usually brains that are
bouncing out of string theory or
bouncing back out you'd call that your
Big Bang but there are issues there as
well concerning how you actually deal
with the bounce itself and all of these
models where you end up dealing with
regions of high curvature where
singularities conform you always ending
up because the mathematics usually is
breaking down on you a degree of
speculations required to to actually
unfelt inner them or not speculation but
but but you need to acknowledge that
that we may not have full control of the
mathematics and that we need to try and
understand that better it's a it's an
amazing idea it's so now let me come to
the bit I said I needed to talk about
which many people would say it places it
on really solid ground which is in many
ways the big key breakthrough or the key
result out of inflation isn't
necessarily this fact that it provides
me with a naturally and natural where to
get a smooth homogeneous isotropic
universe that's not the key thing the
key thing is that it provides me with a
way of obtaining the initial
fluctuations in the matter content in
the universe it provides me with those
seed primordial fluctuations with which
galaxies can then begin to grow which
the microwave background anisotropies
can be observed to be what they are
and the reason inflation does this for
me is this input on this scale of field
again
do you remember I drew the picture in
over here where I said imagine the field
is on a potential which is flat and then
getting steeper okay so and it's
evolving down and I drew it very
smoothly just evolving down but in
reality what's happening is this field
is a quantum field it's not a classical
field like a ball rolling down
it's got this a quantum object quantum
objects inherently have fluctuations
just through the Heisenberg uncertainty
principle so as its mean value is indeed
going down it's wanting to just fall
down all the time it's fluctuating
backwards and forwards backwards and
forwards backwards and forwards these
fluctuations mean that in some regions
of the universe it's slightly higher up
the potential in other regions it's
slightly lower down the potential that
means it's got slightly more energy here
in this region slightly less energy in
this region so now in different parts of
the universe I've got these slight
deviations in energy those feed into the
equations they affect how gravity works
where it's slightly more energy in the
particles slightly less energy in the
particle and it provides you with this
small fluctuations in the pull of matter
towards matter from this region to this
region photons of light experience
slightly different gravitational pulls
as they go through and they lead
naturally to the onset of these
fluctuations which is the macro Cosmic
Microwave Background and our satrapies
which we discussed with about Planck and
then later on from those fluctuations
you seed the structures of the universe
what inflation seems to do quite
remarkably is when you look at the
distribution of these fluctuations of
the microwave background and you look at
these Doppler Peaks okay and then they
can now pick out about seven of these
Doppler peaks and you fit it to your
inflationary scenario it just works like
a dream
it just Maps all of them there are some
slight issues in some regions which
people are homing in on because they're
desperate to try to find something that
might be wrong with inflation but the
odds that we don't yet understand about
it but the general feature seems to be
there that the inflationary prediction
due to these fluctuations of this field
seem to map brilliantly over to the
Cosmic Microwave Background I know so
but and so there's a whole group of
people would say inflation is RIT is on
really firm footing I'm not quite there
because I see inflation as this
wonderful paradigm that works really
well that make the idea works really
well but where did this in photon field
come from where did this scalar field
come from it's got some from from some
fundamental theory and there we begin to
struggle actually if you try to and
that's where I've sort of been doing the
bulk of my work if you try and
incorporate inflation in the world of
particle physics and in our in string
theory it's actually not that easy it's
quite difficult to get a scalar field
which does everything you want it to do
in terms of the Cosmic Microwave
Background and yet also fits very nicely
into the dynamics of the universe from
the world of string theory is it gone
it the bulk of its energy is gone yes
and there are some models of the of the
universe remember I said there are many
models of their simpler time so in the
majority of them is gone that the field
has has decayed and that's the reheating
process at the end of inflation and it's
declared its energy has gone into
creating all the particles but there are
some models called one particular set
models called quintessential inflation
where that that M photon energy hasn't
yet all gone and some of it is still
driving the current acceleration of the
universe the universe is accelerating
again today at a much lower energy scale
than it was accelerating in the early
universe but it still it is accelerating
and some models have that energy there
but the majority of the models would say
it's something else causing that
acceleration today emit and that in
photon energy has indeed gone