Units to cover: 75, 78, 82, 83, 84

Quasars

• Quasars are small, extremely luminous, extremely distant galactic nuclei– Bright radio sources– Name comes from Quasi-Stellar

Radio Source, as they appeared to be stars!

– Can have clouds of gas near them, or jets racing from their cores

– Spectra are heavily redshifted, meaning they are very far away

– Energy output is equivalent to one supernova going off every hour!

• The HST was able to image a quasar, showing it to be the active core of a distant galaxy

Energy Source for Active Galactic Nuclei

• Active galactic nuclei emit a tremendous amount of radiation over a broad range of wavelengths

• A black hole can be both very small, and have an accretion disk that can emit enough radiation

• Likely that at the centers of these galactic nuclei, there are supermassive black holes

• Intense magnetic fields in the accretion disk pump superheated gas out into jets that leave the nucleus

• There are still many questions to be answered…

Seyfert Galaxies

• Seyfert galaxies are spiral galaxies with extremely luminous central bulges

• Light output of the bulge is equal to the light output of the whole Milky Way!

• Radiation from Seyfert galaxies fluctuates rapidly in intensity

Radio Galaxies

• Radio Galaxies emit large amounts of energy in the radio part of the spectrum

• Energy is generated in two regions– Galactic nucleus– Radio lobes on either side

of the galaxy

• Energy generated by energetic electrons– Synchrotron radiation– Electrons are part of the gas

shooting out of the core in narrow jets

The Redshift and Expansion of the Universe

• Early 20th century astronomers noted that the spectra from most galaxies was shifted towards red wavelengths

• Edwin Hubble (and others) discovered that galaxies that were farther away (dimmer) had even more pronounced redshifts!

• This redshift was interpreted as a measure of radial velocity, and it became clear that the more distant a galaxy is, the faster it is receding!

The Hubble Law

• In 1920, Edwin Hubble developed a simple expression relating the distance of a galaxy to its recessional speed.

• V = H d– V is the recessional

velocity

– D is the distance to the galaxy

– H is the Hubble Constant (70 km/sec per Mpc)

• This was our first clue that the universe is expanding!

Which two quantities are shown to be related to one another in Hubble Law?

• A. distance and brightness

• B. distance and recession velocity

• C. brightness and recession velocity

• D. brightness and dust content

Large Scale Structure in the Universe

• Using modern technology, astronomers have mapped the location of galaxies and clusters of galaxies in three dimensions

• Redshift is used to determine distance to these galaxies

• Galaxies tend to form long chains or shells in space, surrounded by voids containing small or dim galaxies

• This is as far as we can see!

An Expanding Universe

• The expansion of the Universe is not like the explosion of a bomb sending fragments in all directions

• Space itself is expanding!

• We can detect photons that appear to have moved at different speeds through space

• Rather, the speed of light is constant, and it is space that was moving relative to the photon

• If each galaxy is like a button attached to a rubber band, an ant walking along the band as it is stretched will appear to have a velocity slower than it really does. The buttons (galaxies) are fixed relative to space, but space itself is moving.

One More Analogy

• The expansion of the universe and the increasing distance between galaxies is similar to the increase in distance between raisins in a rising loaf of raisin bread.

• The raisins are fixed relative to the dough, but the dough expands, increasing the space between them.

• Problem with these analogies – loaves and rubber bands have edges!– We have seen no ‘edge’ to the Universe;

there are an equal number of galaxies in every direction!

– Also, galaxies can move relative to space, as sometimes gravity can accelerate one galaxy toward another faster than space expands!

The Meaning of Redshift

• As light waves travel through space, they are stretched by expansion

• This increases the wave’s wavelength, making it appear more red!

• An objects redshift, z, is

Here, is the change in wavelength, and is the original wavelength of the photon

• This is equivalent to:

=z

galaxiesbetween distance Average

galaxiesbetween distance averagein Change=z

The Age of the Universe

• Thanks to the Hubble Law, we can estimate the age of the universe

• At some point in the distant past, matter in the universe must have been densely packed.

• From this point, the universe would have expanded at some high speed to become today’s universe

• Assuming a constant expansion over time, we find that the age of the universe is around 14 billion years.

Static Universe and Big Bang

Alexander FriedmannDied at the age 27Fred Hoyle

Light from the Big Bang

• Every time we look at the night sky, we are looking back in time

• Can we see light from the Big Bang?– Almost! G. Gamow

A. Penzias and R. Wilson

Alpher, R. A., H. Bethe and G. Gamow. “The Origin of Chemical Elements,” Physical

Review, 73 (1948), 803

The Last Scattering Epoch

• Minutes after the Big Bang, the Universe was opaque– High temperatures kept all matter

ionized– Photons could only travel a short

distance before being absorbed

• After 400,000 years, the Universe cooled enough for electrons and ions to recombine, allowing light to pass

• Now the Universe was transparent!

Light from the Early Universe

• So what should light from 400,000 years after the Big Bang look like?– It should have a spectrum that

corresponds to the temperature of the Universe at that time, 3000 K.

– Expansion of space will stretch this light, however

• The Universe has expanded by a factor of 1000 since this time, so the wavelength will have stretched by the same amount

– Spectrum will correspond to a temperature of 3K.

• This light from the early Universe has been found, and is called the Cosmic Microwave Background

Clumpiness in the CMB

A Timeline of the Universe

The Origin of Helium

• Immediately after the Big Bang, only protons and electrons existed

• Shortly after the BB, temperature and density was high enough for deuterium to form by fusion

• After 100 seconds or so, temperature cooled enough so that deuterium could fuse into helium nuclei

• The temperature continued to cool, and fusion stopped after a few minutes.

• Big Bang theory predicts that around 24% of the matter in the early universe was helium, which matches what we see.

The Epoch of Inflation

• Modern technology allows us to test theories back to a time 10-33 seconds after the Universe Birth (UB).

• Physics as we know it ceases to function at 10-43 seconds after the UB, called the Plank Time

• Using particle colliders, scientists have uncovered a number of clues about what happened in the early universe, after the Plank time

• The early universe underwent a period of very rapid expansion

• By 10-33 seconds, the universe expanded from the size of a proton to the size of a basketball

• This expansion is called inflation

Expansion Forever? Or Collapse?

• The fate of the universe is ultimately controlled by its total amount of energy– Energy of expansion (positive)

– Gravitational energy that can slow the expansion (negative)

– Binding energy

• If the total energy is positive or zero, the expansion continues forever

• If the total energy is negative, the expansion will halt, and the universe will contract and eventually collapse.

Dark Energy

• Dark energy may provide the solution to the mystery

• Dark energy remains constant everywhere, regardless of the universe’s expansion

• Provides an outward push to accelerate expansion

• Dark energy must make up around 70% of all of the energy in the universe

• Much work remains to be done on this frontier…