The Early Universe I

AST 112

Review: Hubble’s Law

• Space is expanding in all directions– This is carrying everything away from everything else– This does not apply to objects that are bound by

gravity

• When light is emitted by an object, it travels in expanding space– Its wavelength gets stretched– It becomes more red

The Age of the Universe

• Galaxy A is 1 MLY (9.46 x 1018 km) away and receding at 22 km/s (6.94 x 108 km/yr). At that speed, how long has it been expanding away from us?

13.6 billion years

• Galaxy B is 2 MLY away and receding at 44 km/s. At that speed, how long has it been expanding away from us?

13.6 billion years

The space in which galaxies live has beenexpanding since the beginning.

Everything has been expanding awayfrom everything else.

If we “rewind”, what happens?

The Early Universe

• Galaxies hadn’t formed when the Universe was significantly more crammed than it is

• But there was still plenty of matter and energy. What would it have been like?

The Early Universe

• Nuclear physicists at Princeton were starting to figure this out

• Hot, dense primordial “fog” of H and He

• George Gamow calculated that the fog in the early universe could only consist of H and He– Not able to fuse into heavier elements

Looking Back In Time

If the Universe used to be a thick, glowingfog of hydrogen and helium, shouldn’t we be

able to see the glow?

(What color is a hot, dense objectat 6000 oF?)

Looking Back In Time

Photons leftover from the hydrogen fog are everywhere. We can observe them.

Why can’t we walk outside and see it?

The Early Universe

• So what do we see when we look at the youngest galaxies?– H and He!

• What do we see when we try to look past the youngest galaxies?– We run into the fog!– The fog is opaque.– The fog is redshifted into microwaves.– So there’s a glowing fog that we cannot see (not in visible light),

into which we cannot see.

• So what do we see?

Looking Back In Time

• Can only look so far back

• Universe used to be opaque – Can’t see light from before then

• Particle physics and computers tell us what happened beyond where we cannot see

An Annoying Hiss, A Pigeon, and the Early Universe

• Arno Penzias and Robert Wilson (1965):

– Set up a sensitive microwave antenna for satellite communications

– Kept finding “noise” in all directions

An Annoying Hiss, A Pigeon, and the Early Universe

An Annoying Hiss, A Pigeon, and the Early Universe

An Annoying Hiss, A Pigeon, and the Early Universe

An Annoying Hiss, A Pigeon, and the Early Universe

An Annoying Hiss, A Pigeon, and the Early Universe

After tightening screws, removing the pigeon,and removing the pigeon again…

The hiss remained!

An Annoying Hiss, A Pigeon, and the Early Universe

• Penzias and Wilson planned to “bury” this nuisance in a paper

• Meanwhile:– Princeton physicists calculated characteristics of

radiation from Big Bang– Should fill the Universe, can see it in all directions– Should be detectable with…

… a microwave detector!

Cosmic Microwave Background (CMB)

• On an airplane trip, Penzias sat by someone who knew of the Princeton calculations

• “Compared notes”: This “noise” in the antenna IS the leftover radiation from the Big Bang!

Cosmic Microwave Background (CMB)

Cosmic Microwave Background (CMB)

• This is an image of “hydrogen fog radiation” over the entire sky

Cosmic Microwave Background (CMB)

• 1990’s: COBE launched– Cosmic Background

Explorer

• The curve is the expected thermal spectrum

• The points are what COBE measured

Cosmic Microwave Background (CMB)

This graph is exactly what we wouldexpect for a single mass of hot plasma

that cooled off.

Cosmic Microwave Background (CMB)

• If you take an antenna-fed TV:

– Turn to a channel for which there is no local station, and you get snow

– 1% of this snow is from photons in the CMB

Receding CMB

• The particles responsible for the CMB are 45.7 billion LY away

• The observable universe ends at 62 billion LY

• As space expands, the CMB will disappear from the observable universe!

The Big Bang

• The two strongest, most cited pieces of evidence for the Big Bang:

– Hubble expansion

– Cosmic microwave background