Hubble’s galaxy classesSpheroidDominates

Disk Dominates

Spiral galaxies are often found in groups of galaxies

(up to a few dozen galaxies)

Elliptical galaxies are much more common in huge clusters of galaxies

(hundreds to thousands of galaxies)

Maps of galaxy positions reveal extremely large structures: superclusters and voids

Models show that gravity of dark matter pulls mass into denser regions – universe grows lumpier with time

Time in billions of years

0.5 2.2 5.9 8.6 13.7

Size of expanding box in millions of lt-yrs

13 35 70 93 140

Structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs

Cepheid variable stars are very luminous

Cepheid variable stars with longer periods have greater luminosities

White-dwarf supernovae can also be used as standard candles

Step 5

Apparent brightness of white-dwarf supernova tells us the distance to its galaxy

(up to 10 billion light-years)

We measure galaxy distances using a chain of interdependent techniques

Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles

The spectral features of virtually all galaxies are redshifted They’re all moving away from us

Hubble’s Law: velocity = H0 x distance

Distances of farthest galaxies are measured from redshifts

One example of something that expands but has no center or edge is the surface of a balloon

Cosmological Principle

The universe looks about the same no matter where you are within it

• Matter is evenly distributed on very large scales in the universe

• No center & no edges• Not proved but consistent with all observations to

date

Expansion stretches photon wavelengths causing a cosmological redshift directly related to lookback time

Critical density of matter

Not enough dark matter

Fate of universe depends on the amount of dark matter

Lots of dark matter

Brightness of distant white-dwarf supernovae tells us how much universe has expanded since they exploded

Accelerating universe is best fit to supernova data

Estimated age depends on both dark matter and dark energy

oldolder

oldest

The early universe must have been extremely hot and dense

Four known forces in universe:

Strong Force Electromagnetism

Weak Force

Gravity

Primary Evidence

1) We have detected the leftover radiation from the Big Bang.

2) The Big Bang theory correctly predicts the abundance of helium and other light elements.

The cosmic microwave background – the radiation left over from the Big Bang – was detected by Penzias & Wilson in 1965

Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3,000 K: visible/IR

Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves

Background has perfect thermal radiation spectrum at temperature 2.73 K

WMAP gives us detailed baby pictures of structure in the universe

Abundances of other light elements agree with Big Bang model having 4.4% normal matter – more evidence for WIMPS!

Mysteries Needing Explanation

1) Where does structure come from?

2) Why is the overall distribution of matter so uniform?

3) Why is the density of the universe so close to the critical density?

An early episode of rapid inflation can solve all three mysteries?

Overall geometry of the universe is closely related to total density of matter & energy

Density = Critical

Density > Critical

Density < Critical

Inflation of universe flattens overall geometry like the inflation of a balloon, causing overall density of matter plus energy to be very close to critical density

Patterns of structure observed by WMAP show us the “seeds” of universe

Observed patterns of structure in universe agree (so far) with the “seeds” that inflation would produce

Many waves ofdifferent sizes,Directions & phases,all “superposed”

Sound waves :red/blue = high/lowgas & light pressure

Water waves :high/low level ofwater surface

“Seeds” Inferred from CMB

• Overall geometry is flat – Total mass+energy has critical density

• Ordinary matter ~ 4.4% of total• Total matter is ~ 27% of total

– Dark matter is ~ 23% of total– Dark energy is ~ 73% of total

• Age of 13.7 billion years

In excellent agreement with observations of present-day universe and models involving inflation and WIMPs!

Finite, But Without Edge?

2-dimensional analogy: Surface of a sphere:

Surface is finite, but has no edge.

An ant has no sense of the third dimension, to him there’s no center - All points are equal.

Any point on the surface can be defined as the center of a

coordinate system.

Expanding Space

Analogy:

A loaf of raisin bread where the dough is rising and expanding, taking the raisins with it.

The Age of the Universe

Knowing the current rate of expansion of the universe, estimate the time for galaxies to move to where they are today:

T ≈ d/v = 1/H ~ 14 billion years

Time = distance / velocity

velocity = (Hubble constant) * distance

379,000 years old: First light escapes; Universe already has structure (light still arriving today)Early fluctuations become denser condensations of matter

First stars form after 200 million years

Galaxies (groups of billions of stars) andgalaxy clusters form, according to the floorplan laid out at 379,000 yearsThe Universe today: lots of stars and galaxies!

Olbers’ Paradox

If universe were

1) infinite

2) unchanging

3) everywhere the same

Then, stars would cover the night sky

Olbers’ Paradox

If universe were

1) infinite

2) unchanging

3) everywhere the same

Then, stars would cover the night sky

Night sky is dark because the universe changes with time

As we look out in space, we can look back to a time when there were no stars