we were in a pub afterwards just in uh
in in near Grandam and uh we were just
finishing off a calculation because we
wanted to show something relate the end
of the UN the end of the universe to um
to dark energy as well that dark energy
is the harbinger of this doom and some
guys were sort of piped off and they
were like what you clearly see us doing
some calculation like what are you doing
we've just shown that the universe is
going to end and they were like what
just
like we're going to start off talking
about something probably 60 people have
heard about before what what have we got
here um cosmological constant problem so
I want to sort of explain what it really
is it's often sort of understated or not
explained properly in um you know well
in lots of places even within scientific
seminars well essentially you think
about um the vacuum you look at a vacuum
and you say right has it got any energy
you think well no it's a vacuum of
course not but actually that's not true
cuz Quantum mechanically what you find
is is that little particles essentially
will pop in and out of existence so you
can think of them as just popping in and
out of existence and essentially you
know from literally infinite decimal
time and that that gives the the vacuum
an energy the question is how much and
it turns out it's an awful lot if you
use your standard calculations to
calculate how much this is you find that
it's a huge amount of energy in fact
it's so much energy that it would cause
the universe to curve so much that it
wouldn't even extend as far as the moon
people have tried to come up with models
that that sort of can suppress the
amount of um vacuum energy that that
that you get out uh none of them have
been particularly successful so then you
sort of face a dilemma well how did the
universe how is the universe so big then
and um and essentially what you say is
okay I've just got a number a quantity
that can cancel off this vacuum energy
and it cancels it it just cancels it off
very ad hoc M manner to a huge degree of
accuracy and that you know sort of this
cancellation then allows the universe to
be to be much larger it's a dimension
full number the thing that you you throw
in there is D has Dimension has
Dimension four dimensions of mass
essentially it's an energy density
itself so you're thinking you throw in
is an energy density and so it's that
energy density you throw in just
literally by hand that you use to cancel
off the energy density of the vacuum and
to try and stop this this huge amount of
curving of the universe but what
actually is it Tony though because when
you explain to me the vacuum energy you
told me what makes it you I know where
it comes from it's it's particles
popping in and out of existence there's
energy therefore we've got this gravity
these are all things that I kind of sort
of understand I don't understand how
you're just throwing this cancellation
number this density where's that coming
from well see actually this this happens
all the time in in in physics um
actually as soon as you start doing
quantum mechanics you can ask what the
quantum Corrections are doing to
something and uh for example the charge
of the electron right you can ask what
the quantum cors are to the charge of
the electron you get all these
Infinities popping out and then you have
to add what we call a counter term to
try and bring it back drag it back to to
finite values and so this is a bit like
that it's like the counter term that you
have to add it seems like a copout
though like if I built my house on a
dodgy foundation and it started leaning
to the side I couldn't just say Okay
number three and everything's better I
would have to actually build stuff and
fix things I would have to physically do
something to fix the deficiency well I I
actually think that's a very good point
BR is that this is that's an excellent
point in fact it sort of always cuts
really to the true nature of the problem
the problem isn't really that you do
these cancellations it's that once you
do them that should be it that that that
choice that you made should be robust
against any change in your your
description of nature if you like so um
so for I just said that you know to
calculate the the the charge of the
electron you're canceling Infinity so
why are we worrying with the
cosmological constant about canceling
huge numbers it's not certainly no worse
than canceling Infinities so and and
that's where the point that that is
generally missed about the cosmologic
constant problem that's not really the
problem the true nature of the
cosmological constant problem is
something called radiative instability
which is this business where you know
let's suppose I calculate what the
vacuum energy is okay so what's my
description of nature well I take a
description that's valid down to the
atomic scale and I get one value for the
vacuum energy and I need to cancel that
so I choose this number to cancel that
now I change by descriptionat I go down
to to the nucleus scale and I get a
completely different answer or and I
need to do the same cancellation to the
same degree of accuracy then I go down
to the subnuclear scale and I get a
different answer again and I need to do
the same cancellation to the same degree
of accuracy and this is a completely
unstable process but surely there was
only one vacuum energy so just figure
out which one you have to calculate and
use that stop calculating it in
different ways okay so that this this it
really hit something quite deep about
about nature do we believe in something
called effective Fai theories that's
that you can literally trust you don't
need to know all these details of these
uh the physics so if you're thinking
about stuff that say super nuclear
scales you don't need to worry about the
details below the sub nuclear scale so
for example do I you know Newton's
constant the law of gravity is that the
value of new constant doesn't change
whether I decide to cut my physics off
at the atomic scale or the nuclear scale
or or whatever right it stays it stays
pretty robust it's the same value the
problem with the cosmological constant
is I'm changing that description I'm
having to keep retuning this value this
value that's really applying on on the
larger scales it shouldn't need to worry
about what's going on at the uh at the
sub nuclear scale and so on and and and
as I move that description once I made
the choice it should be robust and it
really isn't so whenever this happens in
nature it it signals it has always
signaled a lack of understanding of
what's going on we have the same problem
with the mass of the higs incidentally
It suffers from the same kind of problem
that it you know we keep having to
retune retune retune and in that case
super syry solves it case the COS modu
constant well it's a different story you
know what what is going on like this
vacuum energy exists and has a quantity
is it measured or is it something that's
just modeled by people like you
mathematically like we don't know what
it actually what its quantity is are we
just kind of coming up with models
guessing what it is so okay good
question so the vacuum energy does
changes in the vacuum
energy have been measured now what do I
mean by that something called the Casmir
effect where the the value of the vacuum
entry is sensitive to the boundary
conditions of of your say have two
plates and you um you change the the
separation this CH and you can measure
the change in the vacuum energy as you
as you change those boundary conditions
that's not really the true vacuum energy
ground state of the universe it's not
really what just talking about sort of a
localized uh thing there um so and it
really is well you what you actually
measure is is that the effect of
changing those boundary conditions so in
a
sense the vacuum energy the thing that
gives you that is also the same kind of
thing that gives you what what we'll
call the vacuum enage of the universe
the true ground state of the universe so
kind of yes but not quite so so then so
now one asks so you asked about you know
measuring this sort of thing so let's
actually ask how do you measure the
cosmological
constant okay or how do you measure the
vacuum energy well the true vacuum
energy how do you measure it can I
measure it in this
room do you think I can measure it in
this room well I don't know probably not
because of all that noise from the
construction out there yeah right well
you can't right how right so how about
you try and measure you might try and
one of the features of it is it's
constant okay uh as as you go through
space it stays
constant now um well I can look for
something that's behaving constant in
this room but I don't know that when I
open the door and go outside that
changes you just said it was constant so
would the isn't the vacuum energy in
this room the same as it is in Ed's
office of course it is yes it absolutely
is but but when I find the thing that's
something that's constant in this room
how do I know that that's the vacuum
energy it it might not be because that
it might stop being constant when I go
outside the room or when I go outside
the Galaxy or whatever right so or if I
wait long enough it might just change
over time but I just you know I have to
wait long enough to see it see it change
well then you weren't measuring the
vacuum energy absolutely you're not so
then how do you measure the vacuum
energy can you measure in a room clearly
not because you can't distinguish that
scenario well if it can't be
distinguished then it can never be
measured well don't you have to come up
with a vacuum energy me measuring device
right okay so what so what is that okay
the only way to measure the vacuum
energy true vacuum energy is to consider
the entire universe over all time it's
the only way way it's the only way you
can truly guarantee that what you're
talking about is constant this this
observation this is this was due to uh
you know lik niman Hamed Sav deopa big
guns of physics they made this
observation that if you want to identify
the vacuum energy you have to scan all
of space and all of time myself and and
uh colleague of mine nman we we uh we
took what NE and Savas and friends had
had said and we used it we said okay the
only way if the only way that you can uh
measure the vacuum energy is to take the
whole of space and time as one then the
only way you can properly cancel off
vacuum energy is on the same level so
literally we're demanding that this
cancellation happens over the whole
universe that the Universe knows about a
number it needs to know okay um so we've
managed to set up Dynamics which occurs
over the whole universe at once so what
we've done is we've we've got
this number which is the number that uh
you that you use to
cancel and we got another number which
kind of knows about the masses of
particles and on the level of the whole
universe they talk to each other and
they do so so no matter how you
calculate the vacuum energy whether you
work at the atomic scale whether you
work at the whatever
scale those two will communicate in just
the right way that they'll
cancel and so the problem of this this
issue we were talking about just goes
away this paper you've written have you
solved the problem or have you just
written down hey don't worry everyone
the Universe sols on its own so what
we've done is we've found a way to
basically deal with the problem as it
was first formulated that it and it's
it's almost criminally simple it doesn't
require any huge change locally physics
just looks like uh you know Einstein's
relativity Einstein's general relativity
there's no change there why because
we're Chang in we only we change
Einstein's theory on a global level as
the universe as a whole right we don't
play with it locally so locally we get
all the great predictions of Einstein's
theory but the change is done as a whole
because it had to be done on that level
now the um the funny thing is because
we're sort of as described we've got
this way of getting the the number that
you need to cancel this classical value
of the cosmological constant and the
particle masses we get them to
communicate in a certain way
like I said on the universe as a whole
and what that means is it turns out that
the particle masses are are actually
related to the SpaceTime volume of the
universe and in fact as you make the
space-time volume of the universe so
basically it's spatial size and how long
it exists for as you make that bigger as
a whole then the particle masses go down
and in fact if you had an infinite truly
infinite Universe with existing for an
infinite amount of time and over an
infinite amount of space then that would
that would drive all the particle masses
to zero now we know just by looking
around us that the particle masses in
our universe are not zero so that
immediately tells us that if our model
is correct then actually the universe is
going to end we predict Armageddon it's
the main prediction of our um of our
theory that uh the universe inevitably
has to end otherwise we wouldn't have a
universal particle masses in and we do
so so the cancellation of the
cosmological constant is achieved
through it has this KnockOn effect that
the Universe has to be finite it would
predict that the Universe grows from its
initial Singularity grows out of it and
then it reaches a maximum size and then
drives into a big crunch and literally
at the big crunch time and space
completely ceases to exist so time only
exists for a finite finite length of
time otherwise the particle masses would
would have been zero this is if you're
right if we're right yes
yes you do know the mass of the
particles in the universe yes does that
mean you know the number attached to the
cosmological constant what number did
you spit
out okay so in our Theory um there is a
like a residual leftover piece that you
get so the cancellation leaves a little
bit left over and
um that is that that number is smaller
the bigger the universe
gets okay so that we're able to show
that it's less because the universe is
as big as it already is that it's
smaller than than some critical value
that it doesn't screw up any any
cosmology it's constantly ticking it's
constantly changing well it depends I
mean it's it's set it's a number it's
fixed so the universe is going to get as
big as it's going to get right it's not
that it changes over time just that we
know that universe is as big as it is
now so this number can't be any bigger
than this particular value um so you
know yeah what was the original question
what number it's not bigger than the
dark energy density which is what um
it's
a mill electron volts to the fourth so
whatever that is you want that you
wouldn't want that this is a Smoking Gun
to me that I love exact numbers you know
that it's not well I mean we don't know
how how long the universe is going to
last for right so we can't I can't
possibly answer your question I just
know it's not bigger than than the dark
energy density