Lecture 25 The Cosmic Microwave Background Radiation Astronomy 1143 Spring 2014

Testing the Big Bang Our current understanding is that our Universe began in hot Big Bang As the Universe expanded from this hot, dense state, the physical conditions changed Epochs of the Universe Transitions What evidence do we have that the Universe went through these changes?

Idea of expanding space: Aleksandr Friedmann (1922). Observation of expansion: Edwin Hubble (1929). Coining of phrase “Big Bang”: Fred Hoyle (1949). Idea of initial dense “primeval atom”: Georges Lemaître (1931). Who invented the Big Bang Theory?

1 st Piece of Evidence for BB Expansion of the Universe Hubble’s Law + Cosmological Principle shows that space is expanding Necessary for the Big Bang – Universe expanded from a hot, dense state But not sufficient – it is possible to get expanding space without having the Universe change in density – if it is OK for new matter to appear….

Expansion of Universe – 2 Models Big Bang Universe is getting less dense over time Started off hot & dense Steady State Universe is creating new galaxies Always has the same density

The Hot Big Bang What we see Now: The Universe is cold & low density. Galaxies (matter) are getting further apart as space expands between them. As the Universe it expands, it cools further. In the past: Universe was smaller, hotter, & denser Is there any evidence of this early hot, dense phase in the past?

Cosmic Microwave Background Big Bang Nucleosynthesis Neutrino Background Inflation

Key Ideas: Cosmic Background Radiation Relic blackbody radiation from Big Bang Temperature now : T =  K Comes from time when the Universe switched from opaque to transparent (T ~ 3000 K) Electrons & protons combined to form atoms – no more free electrons to scatter light Expansion of the Universe redshifted those photons – peak wavelength now in microwaves We can only look back to the redshift where the CMB comes from

In the early 1960s, two physicists, Wilson & Penzias, were working with a microwave antenna at Bell Labs. (Microwaves are electromagnetic waves with wavelengths from 1 millimeter to 10 centimeters.) Wilson & Penzias were plagued by static. Discovery of the CMB

Wilson & Penzias did everything they could to eliminate “noise” in their antenna. …including trapping pigeons that had left “a white dielectric material” on the antenna. Conclusion: “static” or “noise” actually came from outer space, not from pigeon poop.

Microwave radiation picked up by Wilson & Penzias was nearly isotropic. (That is, it doesn’t come from a single source, like the Sun.) Cosmic Because they come from everywhere, the microwaves from space are called the Cosmic Microwave Background.

The CMB explained The same time that Penzias & Wilson were scrubbing out their antenna, a group at Princeton led by Robert Dicke were building an antenna to look for exactly this signal A friend put Penzias in touch with Dicke Penzias & Wilson won the Nobel Prize in 1978 for their discovery of the CMB Robert Dicke did not ….

Consider the spectrum of the CMB. Measuring the CMB spectrum is hard to do from the Earth’s surface. Water is very good at absorbing microwaves. What kind of radiation is it?

Astronomers observe the CMB from above the Earth’s damp atmosphere with high-flying balloons or artificial satellites. Observations of the CMB

Intensity blackbody The Cosmic Microwave Background has a blackbody spectrum. CMB has a blackbody spectrum

Flashback: Flashback: blackbody spectra are produced by hot, dense, opaque objects. 21,100°F 10,300°F 4900°F

Where did the blackbody radiation come from? The universe is now mostly transparent But it is filled with nearly isotropic blackbody radiation (characteristic of opaque objects). 2.7 Kelvin The temperature of the isotropic blackbody radiation is only 2.7 Kelvin. Very, very cold How can we explain this?

Why is the universe full of isotropic blackbody radiation (the CMB)? very hotvery dense Let’s suppose that the universe was very hot as well as very dense when it started expanding. Hot Big Bang In other words, that the the Hot Big Bang model is correct

If the temperature of the early universe was T > 3000 K, then hydrogen was ionized. Why does this matter? Dense ionized gases are opaque. (You can’t see through the Sun!) Ionized & Opaque

Ionized gases are opaque because they contain free electrons that scatter photons of any energy. Photons (squiggles) don’t move freely through space, because they collide with electrons (purple dots).

Hot, dense, opaque objects emit (blackbody) light! stars Today, we call hot, dense, opaque objects that emit light “stars”. Early Universe was similar to being on the inside of a star (not as exciting as it sounds! The Importance of Being Opaque

The Hot Early Universe The Universe stays hotter than 3000 K for a long time: electrons & nuclei cannot combine to form neutral atoms Universe remains fully ionized Free electrons easily scatter photons Universe is opaque to light during this time. Filled with a hot ionized “fog” of ions & free electrons.

The universe is NOT opaque today. We can see galaxies billions of light-years away. The universe is NOT uniformly glowing today. The night sky is dark, with a few glowing stars.

. Expanding, Cooling Universe Gases cool as they expand. As the hot, dense ionized hydrogen expanded, it cooled When its temperature dropped below 3000 K, protons & electrons combined to form neutral H atoms. The universe became transparent about 400,000 years after the Big Bang

The universe became transparent at a temperature T ≈ 3000 K. visible & infrared microwave But…objects at T ≈ 3000 K produce visible & infrared light (λ ≈ mm) (think “lightbulb filament”), not microwave light (λ ≈ 1 mm). Why Microwave Radiation?

Cosmic Background Radiation After Recombination, the Universe is filled with diffuse, “relic” blackbody radiation. As the Universe expands further: Blackbody photons redshift. Spectrum peak shifts to redder wavelengths, and hence cooler temperatures. By today, the spectrum is redshifted by a factor of ~1000 down to T  3K

. How to “cool” the CMB

Wavelength of cosmic background light has increased by a factor of Why? Why? Because the universe has expanded by a factor of 1000 since the time it became transparent mm 1 mm

inversely Wavelength of peak emission for a blackbody is inversely related to temperature. Wien’s Law – T and wavelength

What is the peak wavelength of the CMB today? T=2.725K

Spectacular Confirmation Current data all spectacularly confirm the predictions of the Big Bang: Perfect blackbody spectrum A single temperature, T =  K Uniformly fills the Universe Details: Fine structure at part in 10 5 level is related to the large-scale structure we see in the galaxies. Currently an object of intense study

Planck Cosmic Microwave Background map 1 part 10 5 roughness

anything The CMB has highest redshift of anything we can see (z ≈ 1000). When we look at the CMB, we look at the surface of the glowing “fog” that filled the early Universe!