# Dark energy

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## INTRODUCTION

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My grandfather wrote a book published 1907 in Germany, where he mentions observations of world collisions. That was what he thought it to be. Today we know these were supernovas.  He mentions among other Nova Persei 1901 and  a supernova from 1885.  Read more in http://www.kinberg.net/wordpress/willy/100_years_later/#nebula

I found July 2021 a great lecture made by Brian Schmidt in 2012, who is “leader of the High-Redshift Supernova Search Team, Brian’s work on the accelerating universe was awarded the 2011 Nobel Prize in Physics, jointly with Adam Riess, Saul Perlmutter and the rest of the Supernova Search Tteam.

In his vide  Brian schmidt “describes this discovery and explains how astronomers have used observations to trace our universe’s history back more than 13 billion years, leading them to ponder the ultimate fate of the cosmos. ” (Source : https://www.youtube.com/watch?v=55pcpTjd3BY )

# INDEX

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## The video

The video is from 2012 but is still very intresting and enjoyable:

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All right, so what is pushing on the universe?

Well, we only have to look to Einstein for the answer.

His cosmological constant, the energy

that is part of space itself, well, that turns out

can actually provide us a way to make gravity push

rather than pull.

This stuff, if it exists, makes gravity push

as the fundamental way that gravity works in his theory

rather than pull.

So by adding some of this stuff to space,

we can go through and get the universe to speed up.

Now, we’re not sure that Einstein’s version of this

is correct.

And so we give it another name, and that name is dark energy.

Now, whenever astronomers use the word dark,

it’s because we can’t see it.

And that means, since astronomers look at things,

we don’t understand it very well.

So dark energy is really “stuff we don’t understand very well”

energy.

So if you do a detailed analysis of our work,

you come to the conclusion that the universe

is a 30% mixture of normal stuff pulling on the universe and 70%

pushing on the universe.

So we really need a little bit of pull, a fair bit of push,

to make our observations make sense.

Now, when we released these in 1998,

the community was justifiably skeptical.

I was skeptical.

I couldn’t believe the universe could be so crazy.

But I knew that our measurements were fundamentally correct,

that the supernovae were too faint to make sense,

except for if something crazy were going on.

So a series of experiments were made.

And the first one was done–

or one of the first ones was done here in Australia,

where a group using the Anglo-Australian telescope,

an Anglo-Australian group, made a map of the nearby universe

out to about a billion light years, making

a map of 221,000 galaxies.

And you can see that the galaxies

aren’t smoothly distributed.

They sort of show this cosmic foam,

and that foam is caused as a signature of gravity.

And so it turns out by looking at this foam

and how galaxies are moving and the nature of this foam,

they were able to very precisely measure

the weight of the universe in gravity as it attracts.

So they actually essentially to weigh attractive gravity here.

And so the amount of gravity pulling on the universe,

by their measurement, was 27% of the amount

of stuff necessary to make the universe flat.

So astronomers weigh the universe, typically,

relative to the amount of stuff necessary

to make the universe just right, to bring it

to that precarious position between finite and infinite.

The amount of stuff in the universe

was 27% of the way there, at least the stuff

that makes gravity pull.

The other experiment that came–

sorry.

But the other thing I need to mention

is that this, while not enough to make the universe flat,

was still five times stronger than the gravity we

could account for by the number of atoms

that were in the universe.

And so this stuff, of course, the shortfall

is what we call dark matter, or in the vernacular

before, “I don’t really understand” matter.

This is stuff which we’re hopefully

going to get an insight into over the next couple of years.

But we think it’s some undiscovered particle that,

like a neutrino, can pass right through the Earth.

So it has gravity, just like atoms,

but is essentially invisible.

That’s at least our hope what this stuff might be.

So the other experiment that was able to be done

was using the cosmic microwave background,

this image of the universe taken right after the Big Bang,

380,000 years back to the Big Bang.

So these sound waves splashing around the universe

have physics which is very similar to what we can do here

on Earth very accurately.

And so the physics tells us exactly how long

these sound waves are.

So for example, one of these sound waves

right here is about 450,000 light years long.

And if you remember, how big something appears

depends on how far away it is.

But it turns out, it also depends

on the shape of the universe.

If you look at things in a curved space,

the light waves get bent.

And so not dissimilar to a car, objects, for example,

in curved space that’s finite appear

larger than in a flat universe.

So we can use that to make a precise measurement

of the geometry of space.

And when you do this, you find that those little bumps add up

to being exactly what you expect for a universe which

is geometrically flat, that is, has 100% of all

the stuff necessary to be flat.

Now, the geometry of space doesn’t

care if it’s made up of stuff that

makes gravity pull or push.

It’s sensitive to everything.

And that allows us to do a little bit of subtraction.

So if we add up everything, we have 100%.

We subtract off the stuff which is attracting, 27%.

And that leaves us with 73% mystery matter,

the same mysterious stuff that the supernovae found,

is pushing the universe apart.

So what does that leave us?

Well, it really leaves us with a mess, a universe where

4 and 1/2% of the universe are atoms, the stuff we know

and love and are made out of.

We represent a very small minority

of what’s in the universe.

The rest of the stuff is dark matter and dark energy,

dark matter pulling, dark energy pushing.

Dark matter pulls along with the atoms

in almost exactly the same way.

Now you might think, well, if we only

understand 4% of the universe, and we

have to make up 95 and 1/2% of the universe,

we just don’t know what we’re doing.

And that may be a good call.

But this model of the universe has been asked

to predict many, many things.

And over the last 13 years, everything

it has been able to predict, we have

been able to go out and measure and show to be true.

And that is how science works.

Reality is what the theory predicts.

You know, when a theory predicts something to be true,

that is the reality of the day.

Now, it may be that there is something

wrong with this model, and we’re getting lucky

being able to predict things.

But the things we predict are sufficiently complicated

now that I think most people think

that this model has, essentially,

the truth embodied in it.

And while it’s probably not a perfect model of the universe,

it is a model like Newton’s gravity,

which works very, very well at describing

the universe we live in.

Crazy?

Yes.

Messy?

Yes.

But it seems to be the way the universe is constructed.

So dark matter.

As the universe expands, the amount of matter and atoms

stays the same, so dark matter’s density

and gravitational effect gets smaller

as the universe expands.

On the other hand, dark energy is tied to space itself.

As the universe expands, the dark energy

gets created with the created space,

and so it becomes stronger relative

to dark matter over time.

So this sets up a battle for domination of the universe,

dark energy versus dark matter.

After the Big Bang, the universe was expanding.

Dark matter would have been very dense and very strong.

It would have been slowing the universe down.

As the universe gets bigger and bigger,

dark matter’s domination is dropping.

And at some point about 5 or 6 billion years ago,

it turns out the universe got sufficiently big,

before dark matter could slow it down,

that dark energy took over.

And so the future of the universe– well,

the future of the universe seems to be dark energy.

The more space expands, the more dark energy

can push against gravity, creating

even more space and even more dark energy,

Eventually, the creation of space

can happen even more quickly than light can travel.

And so galaxies we see today will literally

be lost as their light goes through and is

stranded in the expansion of space

between us and those galaxies.

In the first– in the oldest picture of the universe I

showed you, taken with the Hubble Space Telescope,

those galaxies that we see back 10 to 12 billion years ago–

the light they emit today will never reach us.

Those photons will be stranded in the creation

of space between us.

Now, just to allay some of your fears,

attractive gravity has defeated dark energy

in our part of the universe.

You are not expanding.

The Earth is not expanding.

The Milky Way is not expanding.

And that’s because our part of the universe, dark matter

and atoms overwhelmed the expansion of the universe

13 billion years ago.

And so our part of the universe quit expanding and collapsed.

And there’s a little sphere or ball of material

where there was enough mass to do that,

and that’s what formed our own part of the universe.

However that part of the ball, of the universe,

is gravitationally bound and will eventually

merge into what we will call a super galaxy.

And so we believe the Andromeda Galaxy, which

is one of the few galaxies in the sky that’s

coming towards us, will eventually

merge with the Milky Way 3 or 4 billion years in the future.

And we’re going to have this spectacular change

in the nighttime sky, from first two Milky Ways,

effectively, in the sky, finally merging

into a big ball of stars, into something

that would look more like an elliptical galaxy,

as we call them.

But the rest of the universe beyond that bound ball

will be accelerated out of sight.

We will look out onto stars and nothing else.

The rest of the universe will be empty.

And that will leave cosmologists such as myself,

who study the distant universe and galaxies, out

of a job because there’ll be nothing left for us to look at.

But the reality is until we understand

what is accelerating the cosmos, anything is possible.

One of the most speculative ideas

involves how dark energy might be a little

different than Einstein’s view.

You know, when anything’s possible, this dark energy can change over time and potentially even accelerate the cosmos at a faster rate than Einstein’s version. And this leads to the potential– and I should say, this is very speculative– of something called the Big Rip if dark energy gets created more quickly than the creation of space. That is, if I have a box, and I double the size of the box by the expanding universe, I get more than double the amount of dark energy. Then that leads to a runaway that is able to penetrate to every part of the universe, including your own body, and your atoms, and even down to breaking the universe down into essentially subatomic particles. And this point, as the universe expands more and more quickly, the density of this dark energy rises and eventually approaches  infinity, allowing the dark energy to eat in to where the universe has already collapsed. And so this really has almost a human timescale to it.  As the galaxies disappear– that happened a long time ago.  Suddenly, the stars in the Milky Way will start disappearing. Eventually, the sun would disappear. And then some time later, poof, every atom in your body taken

far enough away that light cannot be transported between any of the atoms, and even the atoms themselves broken apart into quarks and electrons. So a very exciting end of the universe. And that leaves nothing. Well, it leaves something.

It leaves an infinitely dense universe which is expanding very quickly. And that has a certain synergy, I think, with Big Bang. So I kind of like it at some level,but it doesn’t mean it’s true. So this is one of the things we can go out and try to see if the universe is doing. At this point, there is no evidence, unfortunately, that this is going to happen. And I should say, as far as a theory of the universe, it has some real messiness associated with it, having this energy getting created more quickly with space.

However, that aside, really unless dark energy suddenly disappears, the universe will, at an ever increasing rate, expand and fade away in front of our eyes so that people like me, 50 billion years in the future, have nothing left to do.

Thank you very much.

## Notebook of a pluralist

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