The material contents of the universe include about 100 billion galaxies,
that's a hard number to get our heads around.
So lets use the Hubble Deep Field to illuminate this.
The Hubble Deep Field was a time when the Hubble space telescope
stared at a particular patch of sky for several weeks looking incredibly deep
about 10 billion times fainter than the eye can see.
It's worth remembering that the patch of sky observed was tiny about the size of
the head of pin held at arm's length, just a few minutes of arc across.
And yet, this tiny region of space contained 1000
of galaxies at the limit of Hubble's vision.
In this animation, we track across the Hubble Deep Field.
Looking at even a tiny sliver of the field as the galaxy slips by
each one that you see is billion of light years away.
The brightness perhaps, a few billion light years away and
the faintest smudges 8 or 10 or even 11 billion light years away.
Its hard to imagine as we look at these distant stellar systems.
But on some of them,
someone or something isn't doing the same thing and looking back at us.
We have a 100 billion galaxies scattered with large spaces in between in
a universe that's 46 billion light years in any direction.
Those galaxies are made of stars and
the Milky Way contains a typical amount of stars.
There are galaxies both larger and smaller than the Milky Way.
The full stellar census of the universe is ten to the twenty stars.
A 100,000 billion, billion stars.
And since we know that virtually every star is likely to have planetary systems,
you can start to imagine why astrobiologists are confident that there's
life somewhere else in the universe.
The census of atoms in all those stars and
in the spaces between them is 10 to the power 80th.
This is the largest pure number in science.
It's an actual number, the number of atoms in the visible universe.
Most of them hydrogen and helium,
a small fraction of a percent all the other chemical elements.
Strikingly and in a way that can only be explained by modern cosmology and
the big bang theory, there are more photons or particles of radiation in
the universe that atoms by a factor of nearly a billion.
So there are about 10 to the 89th photons in the universe.
They're unnoticeable by us, because they have very feeble energy in our microwaves.
There is nothing about our situation in the extragalactic universe that's
exceptional as far as we can tell.
Our sun is indeed an average star located on the periphery,
but not the very edge of a Milky Way galaxy that is one
of many spiral galaxies we can find with our telescopes.
Our large-scale environment of galaxies is not atypical either.
We are typical or mediocre on every scale that we can inspect.
We're not even made of the stuff that the universe is made of.
Strikingly, dark matter and dark energy whose physical nature is not yet
understood compose most of the universe.
Normal atoms are a tiny fraction.
In fact, the sum of the atoms in those 100,000 billion,
billion stars is just a half percent of what the universe contains.
So the visible stuff, all those stars and
the galaxies is just like the luminous tip of a pyramid of mostly dark stuff.
>> So listen to this.
>> Whenever life gets you down, Mrs. Brown.
And things seem hard or tough [SOUND] and
people are stupid, obnoxious or daft and
you feel that you've had quite enough.
[NOISE]
[MUSIC]
Chris ImpeyDistinguished Professor
