Chapters 16 & 17

The Universe, Dark Matter, Dark Energy

What do we actually know about the Universe?

• 3° K background• Age = 13.7 Billion Years• Pattern of the galaxies is “strange”

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

Dark Matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence

Dark Energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate

Unseen Influences

• “Normal” Matter: ~ 4.4%–Normal Matter inside stars: ~ 0.6% –Normal Matter outside stars: ~ 3.8%

• Dark Matter: ~ 25%• Dark Energy ~ 71%

Contents of the Universe

Evidence for Dark Matter

Rotation curve

A plot of orbital velocity versus orbital radius

Solar system’s rotation curve declines because Sun has almost all the mass

Rotation curve of Milky Way stays flat with distance

Mass must be more spread out than in solar system

From the motions of Milky way stars:

Mass within Sun’s orbit: 1.0 x 1011 MSun

Total Galaxy mass: ~1012 MSun

That is 10 TIMES the mass within the Sun’s orbit!

The visible portion of a galaxy lies deep in the heart of a large halo of dark matter

We can measure the rotation velocities of galaxies in a cluster from their Doppler shifts

What about other galaxies?

Same story as ours…..Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter

The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars!

Thought Question

What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk?

A. Its mass is concentrated at the centerB. It rotates like the solar systemC. It’s especially rich in dark matterD. It’s just like the Milky Way

Thought Question

What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk?

A. Its mass is concentrated at the centerB. It rotates like the solar systemC. It’s especially rich in dark matterD. It’s just like the Milky Way

Clusters contain large amounts of X-ray emitting hot gas

Temperature of hot gas (particle motions) depends on mass….higher temps, more mass

85% dark matter 13% hot gas 2% stars

More Evidence

Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass

More Evidence

All three methods of measuring cluster mass indicate similar amounts of dark matter

• Ordinary Dark Matter (MACHOS)–Massive Compact Halo Objects:

dead or failed stars in halos of galaxies

• Extraordinary Dark Matter (WIMPS)–Weakly Interacting Massive Particles:

mysterious neutrino-like particles

What is Dark Matter..…?Two Options

MACHOs occasionally make other stars appear brighter through lensing

… but not enough lensing events to explain dark matter

• They “weakly” interact with normal matter, like neutrinos– Neutrinos: Travel close to the speed of light, lack electric

charge, have a VERY small mass, and are able to pass through ordinary matter almost undisturbed, they are extremely difficult to detect.

– First suggested to exist in 1930 (from conservation of energy arguments), first detected in 1956.

• They don’t radiate photons• They have lots of mass (Higgs boson)• Interact through the weak nuclear force and

gravity

What are WIMPs?

• There’s not enough ordinary matter…we have to find something….

• WIMPs could be left over from Big Bang

• Models involving WIMPs explain how galaxy formation works

• But…..doesn’t it bother you that we haven’t noticed these things until now…..?

Why WIMPs?

• Gravity of dark matter (maybe) is what caused protogalactic clouds to contract early in time

• Dark matter may hold small galaxies together

What is the role of dark matter in galaxy formation?

WIMPs can’t contract to center because they can’t radiate away their orbital energy

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

What is the evidence for Dark Energy?

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

Accelerating universe is best fit to supernova data

#1: White Dwarf Supernova

#2: The Size of the Cosmic Background Radiation

• The size of “space” and its density seems to exceed the total of normal and dark matter.

• The pattern “seems” to require the influence of dark energy

#3: Galaxy ConfigurationThe pattern of galaxies “seems” to require both dark energy and dark matter

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

• …..we don’t really know

What is Dark Energy?

• Hypothetical form of energy that permeates all space and has strong negative pressure that counteracts gravity

• Homogeneous• Not very dense (10-29 grams per cubic

centimeter)• Does not interact with any force except

gravity!

What is Dark Energy?

• The "cost of having space": that is, a volume of space has some intrinsic, fundamental energy– The energy density of

empty vacuum– As space expands the

density of dark energy remains constant, but the total amount grows

What is Dark Energy?

• “Normal” Matter: ~ 4.4%–Normal Matter inside stars: ~ 0.6%

–Normal Matter outside stars: ~ 3.8%

• Dark Matter: ~ 25%• Dark Energy ~ 71%

Contents of the Universe

Estimated age depends on dark matter and dark energy

oldolder

oldest

The Big Bang: 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 early universe must have been extremely hot and dense

So hot that the distinction between Matter and Energy was meaningless

Photons could transform into matter and back to energy

Photons converted into particle-antiparticle pairs and vice-versa

E = mc2

Every particle has a reaction path like this….

Early universe was full of particles and radiation because of its high temperature

Planck Era

Before Planck time (~10-43 sec)

No theory of quantum gravity

No link between the very big and very small

Universe is the size of a nucleus

Four known forces in universe:

Strong Force Electromagnetism

Weak Force

Gravity

Do forces unify at high temperatures?

Big Bang theory prediction: 75% H, 25% He (by mass)

Matches observations of nearly primordial gases

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

Observed patterns of structure in universe agree (so far) with what inflation should produce

“Genetic Code” 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!

Big Questions…

• How did the Universe begin?• Why is it expanding?• What is the fate of the Universe?• Are there more dimensions? • Is there life elsewhere in our solar

system?• Is there intelligent life elsewhere in the

Universe?