Cosmology

“This is the way the world endsThis is the way the world endsThis is the way the world endsNot with a bang but a whimper “

T.S. Eliot, The Hollow Men (1924)

Hubble's Discovery

Discovered:

•Recession velocity gets larger with distance.

•Used cepheids for Andromeda and Local Group

1929: Edwin Hubble measured distances to 25 galaxies:

•Compared distances with recession velocities

As the Universe get 2x larger, the distances between galaxies get 2x larger but, the sizes of the galaxies remain the same. Galaxies are bound together by gravitation.

The Universe is expanding !

Hubble Law

v = Ho d = cz where

v = velocity from spectral line measurements

d = distance to object

Ho = Hubble constant in km s-1 Mpc -1

z is the redshift

Hubble Parameter: H0

Measures the rate of expansion of the Universe today. H0 = 71 km/sec/Mpc

•Best measurement to date is from the Hubble Key Project to measure Cepheids in nearby galaxies.

H0 is very hard to measure •Recession speeds are easy to measure from the shifts of spectral lines. •But, distances are very hard to measure.

Age of the Universe?• Hubble’s constant converts to an

age for the Universe

• Corrections (via Cephid observations push H to 70, and the age to ~14 billion years

12110 yearsT

H

•If H >80 then there are globular clusters that are older than the Universe, not possible.

Cosmological Constant

• General Relativity automatically predicts that Universe should be either contracting or expanding.

• Einstein didn’t know that this was happening and added a term to his equations to make the Universe stable.

• The term is called the cosmological constant, and Einstein set it = 0. He later said that this was his greatest blunder.

Two ModelsMany cosmological models have been proposed, but the

two main theories are:• 1) Big Bang Model based on Einstein's general theory

• 2) Steady State Model which requires some changes in standard relativistic ideas.

• The universe is expandingWe have observed this for some time.

Principles of CosmologyHomogeneous – Matter is spread uniformly through space

Isotropic – The Universe looks the same in every direction

Universality – The basic principles of physics are the same everywhere

The Cosmological Principle: The universe will look more or less the same to observers everywhere, in every galaxy, no matter where it is

What holds the universe together?

•Gravity – very weak, but operates over incredibly long distances.

•Electromagnetism – strong, but operates only over moderate distances

•Strong force – the strongest, but only good for about 10-15 meter. Keeps nuclei together

•Weak force – regulates radioactive (fission) decay.

Unified Forces

Big Bang--a Better, Simple Model• The expanding universe that resulted from the big bang

is most accurately thought of in simple terms as the surface of an expanding balloon with small dots on the surface.– the surface of the balloon expands (spacetime)– all galaxies on the surface move away from each other. – the surface of the balloon has no edges!

2D look

The Three Pillars of the Big Bang

Three fundamental pieces of evidence: Expansion of the Universe:

•Explains the observed Hubble Law •Age of the Universe consistent with the ages of the oldest stars.

Primordial Nucleosynthesis: •Creation of the original complement of light elements •Deuterium (2H), Helium, & traces of Li, Be

Cosmic Background Radiation: •Relic blackbody radiation from the early hot, dense phases of expansion.

Penzias and Wilson• Mid 1960’s Penzias and Wilson detect a

uniform isotropic radio signal that couldn’t be explained .

• The spectra of this signal fits a black body curve for an object of T=2.74 K

• It is described as the

• Cosmic Microwave

• Background Radiation.

(Bell Labs)

in 1988 for their discovery.

They won the Nobel Prize

Cosmic Microwave Background• Big Bang theory predicts that the early

universe was a very hot place and that as it expands, the gas within it cooled.

• Thus the universe should be filled with radiation that is literally the remnant heat left over.

• Since this radiation has cooled since it was formed, it is now at a temperature of 2.73 K

• Radiation of this temperature radiates in the microwave region– hence the Cosmic Microwave Background

Mollweide Projection

The above image has a typical dipole appearance because our Galaxy is moving in a particular direction. The result is one side of the sky will appear redshifted and the other side of the sky will appear blueshifted.

The raw image of the sky looks like the following:

•COBE’s measurements of CBR.

Processed COBE Data reveals structure in CMB

Moving towardsblue, away fromred (Doppler shift) at a speed of 368 km/s

Dipole amplitude is one partper thousand of 2.7°K.

Removing the Galaxy's motion produces the following map dominated by the far-infrared emission from gas in our own Galaxy. The gas is the dark red strip around the equator.

•Removing the infrared emissions from gas in our own Galaxy, the cosmic background radiation is distributed as predicted and has been confirmed by multiple surveys.

Cosmic Background Radiation

Misconceptions

•Another misconception - Singularity as a little fireball appearing somewhere in space.

•The singularity didn't appear in space; rather, space began inside of the singularity. Prior to the singularity, nothing existed, not space, time, matter, or energy - nothing.

Big Bang Theory -We tend to imagine a giant explosion. Experts say that there was no explosion; there was (and continues to be) an expansion.

There is no “Center”

• The Big Bang involved the entire universe. Every place in it was at creation, which was an event marking the beginning of time. There was no center because there was nowhere to measure it from.

The Chain of Events

• We can not answer the question: “What was the Universe like before the Big Bang?”

• We can not answer the question “What was the Universe like in the first 10-34 seconds ?”

• We can however speak of conditions in the early universe after that time…

The Chain of Events

• In the earliest Universe, the temperature was so great, that the universe was mostly gamma rays (radiation dominated).

• The gamma rays could spontaneously decay into almost anything (massive particles or light particles) as long as both a matter and anti-matter particle were created.

Matter vs. Antimatter• Matter vs. Antimatter

– For every particle, there is an antiparticle of opposite charge

– Antiparticle examples• Positron (opposite of electron)• Antiproton (opposite of proton)

– Matter and antimatter annihilate each other

• E = mc2

• Energy condenses into matter• Matter annihilates to create energy• Mass + energy are conserved

– Universe is matter-dominated• What happened to the antimatter?• Is matter slightly longer-lived?

• As the Universe expanded from this singularity, it cooled.

–Photons became quarks

–Quarks became neutrons and protons

–Neutrons and protons made atoms

–Atoms clumped together to make stars and galaxies

Formation of Galaxies• Galaxy formation began

in the first billion years of the universe

• Elliptical galaxies form stars at a faster rate, possibly preventing the collapse of the proto-galactic cloud into a disk

• Slower star formation rate allows disk to form leading to spiral galaxies

If we run the clock backwards far enough, eventually the Universe would be zero size and therefore infinite density and infinitely hot. We call this very hot, very dense initial state the "Big Bang"

•The Universe must have begun as a singularity.

•The theories of space and time break down before the singularity at a time known as the Planck time

Multi-universes

• Universe was originally defined to include everything

• However, with inflation, the possibility exists that our “bubble universe” is only one of many such regions that could have formed

• The other universes could have very different physical conditions as a result of different ways that the unified symmetry was broken!

The fate of the Universe• How will the Universe end???

• It all depends on gravity…

•Galaxies are moving away from each other, and gravity is pulling them back together

•Is there enough gravity to stop the expansion and pull everything back together?

•Mass is the measure of how much gravity something generates

• There are 3 possibilities– The expansion will continue forever

– The expansion will slow down and stop

– The expansion will slow down, stop, and reverse

The Fate of the Universe

•The answer depends on only one thing:

–Is there enough Matter in the Universe

–To generate enough Gravity

– To stop the expansion?•The amount of matter needed to just bring the expansion to a halt is called the Critical Density.

• If the density of the Universe is larger than the Critical Density, then there is enough mass to stop the expansion,and to pull it back into a re-collapse “Big Crunch”

• If the density of the Universe is less than the Critical Density, then there is not enough mass to stop the expansion, so the Universe expands forever!

• If the density of the Universe = the Critical

Density, the expansion slows to an eventual stop. This is essentially the same as “goes on forever”

Only the rate of the expansion differs

1M

High density

Low Density

Intermediate Density

1M

0.3M

Unbounded

Marginally Bounded

Bounded

Expansion rate now is now regardless of past or future. This is also the slope of the curves. Today it is 71 km/s/Mpc

Obviously these 2 futures are different

• High-Density– As the Universe contracts– It gets smaller, hotter, denser– Ultimately ends in a “heat death”

• Low-Density: the Universe expands– The rest of the Universe will be too far

to see– All Stars will eventually run out of fuel– Ultimately ends in a “cold death”

Type Ia SN results indicate that the universe is accelerating.

But wait, something new !

Recent WMPA background cosmic radiation data implies that , so 0.7 must be something besides matter.

1.0 .02

decelerating.

Deceleration => the expansion rate is slowing

Acceleration => the expansion rate is increasingcontrary to our initial bias that it should be

Dark matter in clusters

• Galaxy clusters– Lots of dark matter between the

galaxies

– Motions of galaxies in clusters• Measure speeds of galaxies

• Find mass/gravity needed to hold onto them

•As of this time, galaxies + dark matter are not enough to ‘close’ the Universe

•The Universe will continue to expand forever

Is the Universe Flat or Curved

• Bound (closed)– spherical

geometry

• Marginally bound– Flat

• Open– Hyperbolic

Observational Data

• Data not yet conclusive, but universe is very nearly flat!

• Recent data from supernovae observations indicate expansion may be speeding up! Verdict still out.

Flatness Problem

• Inflation flattens out spacetime the same way that blowing up a balloon flattens the surface

• Since the Universe is far bigger than we can see, the part of it that we can see looks flat

What is inflation?The early huge rate of expansion of the universe has come to be called inflation. At the end of this epoch, elementary particles such as electrons, protons formed.

•Inflation is a prediction of grand unified theories in particle physics that was applied to cosmology – it was not just invented to solve problems in cosmology

Accelerating Universe • Recently, Type 1a SN observations show

that the expansion rate of the Universe has not been constant – i.e., that the Hubble constant is not constant

•Recent observations seem to show:

–The expansion seems to be speeding up

–There may be a repulsive force that acts at great distance, pushing galaxies apart?

•Known as dark energy

Dark EnergyDark EnergyDark EnergyDark Energy

•There is some type of dark energy which is causing the expansion of the Universe to accelerate. The universe is accelerating outward and it will expand forever.

Acceleration and Deceleration

• Evidence from HST supernova observations indicates that the expansion of the Universe is now accelerating, but that it was decelerating in the past as matter formed and gravity dominated

Wilkinson Microwave Anisotropy Probe (WMAP)

• Launched June 30, 2001. Measured fluctuations in CMBR on a scale of 0.2 - 1 degrees (vs. 7o for COBE) and filled in the fluctuation plot

Key results:

•Hubble constant is 71 km/sec/Mpc (to within 5%)

•Age of the Universe is 13.7 billion years old (to within 1%!)

• First stars appeared at t = 200 million years after the BB

• Universe is geometrically flat

Contents of the UniverseWMAP cosmology

•of structure in the universe. They would have prevented the early clumping of gas in the universe, delaying the emergence of the first stars.

Fast moving neutrinosdo not play any major•role in the evolution

Dark matter acts as a gravitational force, or a breaking force on expansion.

Dark energy is a repulsive force, or a gas pedal for expansion.

Early in the Universe, matter was close together, and gravity was slowing the expansion.

When the Universe was 5 by old, matter was spread thin enough, that dark energy started the Universe to accelerating. This change is reflected in the redshift of galaxies at different distances.

So what is the final fate of the Universe ? That depends on what dark energy turns out to be.

If the amount of dark energy is constant, as it expands, density will drop, and expansion will slow as gravity puts on the brakes. We are back to our choices of closed. Open, or flat.

If the density of the dark energy stays the same,the expansion rate stays the same. In this case matter would spread out, and space would be cold , and empty.

If the acceleration speeds up, disruptive forces would begin to tear galaxies, stars, planets apart. This is called the Big Rip.

So, we need to know, what is dark energy ?

The End

Structure of the Universe• Cosmology

– Cosmology is a description of the universe

– Different Cosmologies• Geocentric Universe – Ancient Greeks

• Heliocentric Universe – Copernicus

• Newton – Infinite Universe– Olber’s Paradox

• Herschel – Milky Way

• Current cosmologies– Evolving Universe (Louis Friedmann)

– Steady State (Sir Fred Hoyle)

– Big Bang (Georges Lemaitre)

– Inflation (Alan Guth)

– Variable Speed of Light (João Magueijo )

• Size of the Universe– Visible universe limited by the speed of light

– 2003 measurement places size of visible universe at 13.70 ± 0.14 billion Lyr across

– Most distant galaxies and quasars are literally at the edge of the visible universe

Inflation Theory• Inflation Theory

– Universe originated in the Big Bang

– Problems with Big Bang• Flatness problem –Universe looks flat today, but it should not! If Universe had even a tiny deviation from

perfect flatness, that deviation would have become more pronounced with time. • Horizon problem – Distant regions of the universe appear to have exchanged information even though

light has not had time to cross the distance between the regions

– Inflation theory – suggests that the universe expanded from the size of an atom to the size of the solar system in a tiny fraction of a second

• Allowed because it is space-time itself that is expanding. Speed of light only limits the exchange of information through space-time, not the rate of space-time expansion

• Flatness problem is resolved because universe expands so fast that curvature is too small to notice on local scale. Think of humans on Earth’s curved surface. Because the curvature is so small, we perceive that Earth is flat.

• Horizon problem is resolved because two regions of space could touch before inflation, then spread apart faster than the speed of light.

– Matter-Antimatter asymmetry is not handled by inflation theory

Fate of the Universe• Open Universe

– Less than critical density– Expansion overwhelms gravity– Universe expands forever– Heat death

• Static Universe– Exactly critical density– Expansion stopped after infinite time– Heat death

• Closed Universe– Greater than critical density– Gravity overwhelms expansion– Expansion stops in finite time– Universe collapses in a Big Crunch

• Accelerating Universe?– Observations show universe is not slowing

down or even coasting– It seems to be accelerating - dark energy

• Critical Density– Does universe have enough mass for gravity to

stop the expansion?

– We are very close to the critical density

– Arguments suggest that we would not be here if we were not near the critical density

History of the Universe

Fate of the Universe

Planetary systems known to date• 146 planetary systems discovered in last 10 years

– 170 planets total– 18 multi-planet systems

• Discovered by seeing star wiggle under gravitational influence of planet– tends to find BIG planets CLOSE to the parent star (biased)

sun’s path in 65 years

red points areindividualmeasurements(with errorbars)

black line isbest-fit ellipticalorbit

8 MJUP at 2.88A.U., 0.29 ecc.

“Trajectory” of Expansion• Orange: Closed; re-

collapse• Green: Flat; teeter• Blue: Open; eternal

expansion• Red: our universe; flat,

but accelerated

what we know at present is the slope—which is why the curves above all have same “Now” slope

The New Picture

• Dark Energy messes up this picture– though at critical energy density, not all in form of

matter– not enough gravity to halt expansion– being accelerated to boot!!

• Best guess as of now: eternal, accelerating expansion– shrinking horizon (ultimately less of universe visible)– called the “cold death”—universe continues to cool as

it expands

COBE’s Great Finding: Anisotropy in the CMB

Red means cooler, blue means warmer, but only by tens of micro-Kelvins.COBE’s vision was limited to 7 degree resolution, but we see structureat this scale, representing density variations in the recombining plasmawhen atoms first formed. These density variations reveal the seeds of

galaxy formation. This is as far back as we see—the wall to our vision.

Next-Generation CMB Experiments

• BOOMERANG: Antarctic Balloon-born experiment.– Flew a 10-day circle around the pole in 1998

• returned within 50 miles of launch point!

– Mapped small (3%) part of sky at high resolution– Seeking characteristic scale of fluctuations

BOOMERANG Results: Anisotropy BaredPatch of “blank” sky ~30 on

a side: structure about 1° scale.

This structure is real: any expmt.to follow will find same imprint.It’s our cosmic wallpaper.

These are the seeds of galaxyformation.

Fluctuations are at about one ortwo parts in 10,000 relative to the2.7°K background.

Size of fluctuations tells us aboutthe geometry of the universe.

BOOMERANG: Our Universe is Flat Top: BOOMERANG

Bottom: theoretical expectations

Know the true physical size offluctuations from plasma physicsand early cosmology (when theuniverse was far simpler than now).

Know how far away the structure is.

Apparent size of structure affected bygeometry of Universe: flatnomagnification; positive curvature(closed)looks bigger (magnified);negative curvature (open)lookssmaller (like through binocularsbackwards).

Which looks right to you?

Pre-BOOMERANG State• This complicated plot shows

where measurements stood in 1999

• Theorists predicted a sort-of ringing structure (black curve) to the CMB bumps

• The data (with error bars) are all over the place!– note COBE only pertains to the

crudest (largest) structures

• My reaction at the time:– the theorists are nuts—no way

will we see this funky structure: the real universe will surprise us all

• Note how many independent teams are chasing after same goal

10 scale 1 scale 0.1 scaleall-sky

WMAP Adds new results (2003)

Whole-sky map at BOOMERANG qualityExact match to BOOMERANG map

This is our wallpaper—we’re stuck with it

COBE vs WMAPLimited resolution

Only hints at truth

Fully resolved

COBE structurestill present

And the Result…

WOW! Whatever the theorists were smokin’, it put them right on target!

The Age of Precision Cosmology

age of U

hubbleconstant

age atCMB

total density

dark energy

baryons

total matter

Meanwhile, Supernovae Indicate AccelerationFarther Away

Closer

Distant supernovae (“standard candles”) appear farther away than they “should.”“Should” represents the case wherein the Hubble expansion is linear,or unaccelerated. That is, redshift is strictly proportional to distance.

“Should” liealong this line.

But they’re justa bit higher.

Farther awaythan expected.

Expansion isaccelerating!

Putting It All Together• Based on CMB structure scale:

– Universe is flat– parallel lines remain parallel forever

• Together with supernova studies, other CMB studies:– See plaid checked region…– Universe is accelerating!– Cosmological Constant, – Vacuum repulsive energy

• Two independent (and competing) groups found same result– dashed vs. solid ellipses

• Clustering of galaxies separately indicates M 0.3

Matter Density

Den

sity

fro

m “

Cos

mol

ogic

al C

onst

ant”

More recent update (better data)• X marks the spot

– The only place where all three measurements agree is with M0.3 (matter) and 0.7 (dark energy)

denotes a density M=1 means critical density:

enough matter to halt expansion of universe

• There’s not enough matter to stop the expansion

• Worse still, the dark energy accelerates the expansion

• Theorists wanted M=1.0 and =0.0

The Mass Budget of the UniverseThe Mass Budget of the Universe

Most gravitating matter is dark (not in the form of shining stars).Most of this, even, isn’t in a familiar form of particle (non-baryonic).

Most of the energy density in Universe isn’t mass at all!

“Critical” Density

“Best Guess” Density of matter in the universe

Stars: less than 1% of critical density

Ordinary (“Baryonic”)Matter, at most 5%

of critical density

“Dark Energy”,Cosmological Constant

Cosmological Conclusions• Universe is expanding

– We’ve known this since the 1920’s.

• Not enough matter to gravitationally arrest expansion– Only about 30% of the necessary total

• Evidence that expansion is in fact accelerating– Other 70% of Universe’s “density” may be pushing

• Most of the gravitating matter is in a form as yet unidentified– 23% out of 27% = 85% of gravitating matter mysterious– ordinary nuclei (called baryons) ruled out– probably some exotic new form of matter

• No convincing ideas for the nature of the “dark energy”– scalar fields, cosmological constant, GR wrong or needs modified?

Lecture 1 A Brief History

A modern Hubble diagram, made using galaxies with a detected Type 1a supernova

Cosmological Conclusions• Universe is expanding

– We’ve known this since the 1920’s.

• Not enough matter to gravitationally arrest expansion– Only about 30% of the necessary total

• Evidence that expansion is in fact accelerating– Other 70% of Universe’s “density” may be pushing

• Most of the gravitating matter is in a form as yet unidentified– 23% out of 27% = 85% of gravitating matter mysterious– ordinary nuclei (called baryons) ruled out– probably some exotic new form of matter

• No convincing ideas for the nature of the “dark energy”– scalar fields, cosmological constant, GR wrong or needs modified?

Cosmological Conclusions• Universe is expanding

– We’ve known this since the 1920’s.

• Not enough matter to gravitationally arrest expansion– Only about 30% of the necessary total

• Evidence that expansion is in fact accelerating– Other 70% of Universe’s “density” may be pushing

• Most of the gravitating matter is in a form as yet unidentified– 23% out of 27% = 85% of gravitating matter mysterious– ordinary nuclei (called baryons) ruled out– probably some exotic new form of matter

• No convincing ideas for the nature of the “dark energy”– scalar fields, cosmological constant, GR wrong or needs modified?