How the Universe got its SpotsHow the Universe got its Spots

Edmund Bertschinger

MIT Department of Physics

Outline

• Introduction: the Big Bang

• Cosmic Microwave Background Radiation

• A sonogram of the early universe

• What have we learned?

• Outstanding questions

NASA/WMAP Science Team

The Big Bang

• The Universe as we know it began 13.7 billion years ago in an explosion so powerful that space itself was propelled outwards.

• Even atoms could not exist in this hot fireball.

• 380,000 years after the bang, H atoms formed

• 200 Million years later, the first stars formed

How do we know?

Measuring the Afterglow of the Big Bang

WilkinsonMicrowaveAnisotropyProbe(WMAP)

Launched:Summer, 2001

(NASA/WMAPScience Team)

The Cosmic Microwave Background RadiationFrom COBE (1994) to WMAP (2003)

NASA/WMAP Science Team

What is the Microwave Background?

The cosmic microwave background radiation is the thermal (blackbody) radiation emitted by the hot dense gas formed in the Big Bang.

It was discovered by Penzias and Wilson in 1965; they received the Nobel Prize in 1978.Animation

WMAP Actual Sky Maps

Top: Foregrounds removed. Bottom: 41 GHz

NASA/WMAP Science Team

Our View Before WMAP

A cosmic sonogram

• The Big Bang began with a burst of near-exponential expansion: cosmic inflation

• Quantum mechanical fluctuations in energy made ripples in the gasAnimation

• Fluctuations in radiation == Sound waves!– Light as sound?! Yes!

• Cosmic redshift: Doppler shift in an expanding universe

The Sonogram in Numbers:Angular Power Spectrum

Top: Temperature fluctuations vs. angular scale

Bottom: Cross-correlation of temperature and linear polarizationvs. angular scale

What have we learned from these spectra?

• Age, size, and curvature of the universe– 13.7 Billion yr, consistent with infinite Euclidean space

on scales of 50 Billion lt-yr

• Contents of the universe: bizarre!– 4% atoms (almost all Hydrogen and Helium)– 23% cold dark matter (elementary particles as yet

undiscovered on Earth) detected by its attractive gravity– 73% mysterious “dark energy” (we’re not even sure of

the physics of this substance!) detected by its repulsive gravity causing the cosmic expansion to accelerate

• Hydrogen was re-ionized about 200 Myr after BB

We are not made of the same type of matter as most of the Universe!

The WMAP results greatly improve our estimates of cosmic parameters.

Four years ago, cosmologists proposed an accelerating universe.

WMAP (in green) strongly confirms this.

NASA/WMAP Science Team

We have seen the seeds of cosmic structure

animation

Outstanding questions

• What is the dark energy?– Is it the “cosmological constant?”– What new physics acts on cosmic scales to

produce this substance?

• What is the dark matter?

• Can we now fully understand the formation of galaxies?