Okay, welcome back everybody and so here we go, we can answer the,

the question for this week which is, what is the fate of the universe.

Well, the answer is, we don't know, it all depends, it

all depends on sort of you know, the properties of things like

the dark energy which we are not entirely clear about, but we

can certainly articulate the possibilities of how the universe will go on.

One is the big crunch.

Right?

One is the possibility that the universe will actually expand

for a certain amount of time and and then stop expanding.

And then, collapse back on itself.

Into a you know, what would be called a big crunch.

And then, you know, we'd basically

be driving ourselves into another singularity.

And what would happen after this?

Would it be a big bang or not?

It's very unclear.

Classical, one of the problems with this is that relativity,

general relativity, Einstein's theory of space time is a classical theory.

There's no quantum mechanical theory for it as we've talked about before.

And so, a lot depends on the

actual resolution of the singularity with quantum mechanics.

So you know, there are some ideas out there that sort of can show that if you do

through a big crunch, the atoms of space time,

so to speak, keep you from having crazy things happen.

And you just bounce, basically, into the next universe.

Another possibility is sort of like what you might think

of as sort of the heat death of the universe.

Or just you know, sort of the, the, the death, death by boring.

By boring this.

Which is that the universe continues to expand forever, and in its expansion

basically you run out of interesting things that can happen.

So basically, you run through all the mass that's

going to go into stars, stuff turns into stars, and either

becomes a black hole at the end of its lifetime,

or becomes a neutron star, or becomes a white dwarf.

And that's it, that material, those are dead

cinders of stars and they just basically sit

there forever, there's, if you go for very

long times, even the matter itself may become

unstable but you basically end up with a universe that is dark, that is, there is

no more light being produced and so You

know, and here's the interesting thing to think about.

If the universe does expand essentially infinitely.

Then the 13.7 billion years, which is 10 to

the approximately 10 to the 10 years is nothing.

This is still, there's going to be 10 to the 100 years, 10 to the 1,000 years.

Right?

And so, most of the universe's history is going to be boring with nothing happening.

It's going to be dead and dark so it's a kind of a, and thin, right?

Because matter of the universe will continue to expand and so

there won't be a whole lot of stuff to even see.

The galaxies, you know, everything will be over each others'

horizons, you won't even be able to essentially see anything anymore.

A more dramatic, though nonetheless horrific ending, is that

the nature of the dark energy is such that things

continue to expand, and expand faster, and expand faster,

and expand faster and that the acceleration goes on forever.

In what we would call a, a big rip,

that the acceleration of the universe goes on so a,

so rapidly eventually that it'll eventually be expanding so

fast that everything is carried over beyond the horizon And

that everything, you know, galaxies will be torn apart

by the expansion of space, the, the, the gravitational traction

which holds galaxies together will be overwhelmed by this ongoing

acceleration of the expansion of space such that eventually galaxies

will be torn apart.

And then stars and planets will be torn apart.

Even atoms will be torn apart.

So this is what's called the big rip.

And then eventually, you know, you end up

with space that is essentially empty and completely flat.

And that's it, there's no way, you know, of ending.

Now of course, you know, what you always can end up

with these cases where you end up with space being essentially flat.

Some people think about new quantum fluctuations that

lead to new periods of inflation or something.

These are all very speculative ideas but you know, the whole idea of

the long term history of the universe is something that is inherently speculative.

So we don't know essentially when a, you know this would occur,

we don't know how any of this is going to occur, we don't

really know what the answer is and it leads us to, probably the

way to sum up this course, which is thinking about ultimate questions, right?

Some of the questions we've addressed are not ultimate.

How does a star form?

Well that's a question that fits very much

neatly within the, the parameters of traditional science.

It's there's the laws of physics we understand.

We understand how they combine together.

They tell a very good story that could be very tested.

And you know, there we go.

We understand how stars form and how they die.

An ultimate question is a question like where did the

universe come from or where is it going to go.

And, these ultimate questions are the

place where science and, you know, religion

kind of bounce up against each other, and it's not clear to me

whether or not, you know, science will be able to provide an answer

any more than, you know, theology provides an answer, they're not really answers.

What they are are your own biases.

You know, your own reasons for thinking that it's this way.

One way or another.

Some people will say that science actually will be able to provide ultimate answers.

And that's the interesting thing.

We'll see, as time goes on.

But what is most important is that science

gives us a way of addressing these questions.

In a way that is has the, the hope of formulating

the question logically, asking what kind of evidence there is for it.

And so we can distinguish between whether or not we have answers

that are amenable to interrogation or whether we are just left with

the mystery and there's nothing wrong with mystery, don't get me wrong,

mystery is a wonderful thing, human life is surrounded by, by mystery.

We should never give up on it, we should never think

that science is some way to eradicate mystery from our lives.

But what is beautiful about science and

these big questions that science answers, is

it allows us to approach those questions with a, with some of our best talents.

Right our, our ability to ask questions, our ability to formulate testable answers.

So throughout this course we've looked at a, a number of different

big questions, some of which are ultimate questions and some of which aren't.

But what's most important, and what I hope you take away from this course, is the the

beauty of the stories that come from our

encounter with these a, a, these questions and how

much progress humanity has made oh, in the last 2,000, 10,000 years

in being able to just go from just imagining mythologies You

know, perhaps based on some intuition, to actually developing narratives that

have the possibility of being tested.

They are the, the mythologies are beautiful but the

stories that we can actually test are even more beautiful.

So, with that, I thank you very much for your time, for being part of

the course and I hope you go further and read lots of books on this subject.

Okay.

Answering the Question

Confronting The Big Questions: Highlights of Modern Astronomy

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