Astronomy 101 Final exam:

Time: Monday May 19 at 8:00 AM

Place: Gym (not the Sports Complex, and not the Dance Studio)

Final grades will be assigned on the basis of the total course grade:

Each midterm: 15%

Final Exam: 30%

Recitation: 30%

Projects: 10%

We will apply a curve to this final score to obtain the course grade.

See a total eclipse of the moon, Thursday May 15, around midnight: info at http://skyandtelescope.com/observing/objects/eclipses/article_923_1.asp

On August 27, Mars and Earth will be closer together than for 57,000 years. The next record approach is in about 280 years , so plan to catch this one !

http://skyandtelescope.com/observing/objects/planets/article_929_1.asp

Optional review, Monday May 12 in lecture hour (10:30 to 11:25) here

Observing project reports – returned today at entrance to lecture hall

Astronomy 101 Lecture 28, May 7 2003

The early universe – (Chapter 27 in text)

OK, we believe in the Big Bang around 13.7 billion years ago.

What happened in the very earliest moments, days, years of the universe, and how did those events shape our world today?

The environment in the early universe was changing very rapidly: from the initial point ‘singularity’ of the big bang, as the universe expanded, the Temperature dropped and the density of matter and radiation (photons) decreased.

The universe passed through a series of epochs with differing conditions, depending on the Temperature.

Nowadays, we see density of about 2 x 10-27 kg/m3 of matter. The radiation is dominated by the blackbody cosmic microwave background (starlight is negligible), and the density in mass terms (E=mc2) is only about 10-32 kg/m3. So we are in presently in a matter-dominated universe; matter density exceeds radiation density by about 100,000.

Going back toward the Big Bang, the smaller universe means that both the density of atoms/protons/electrons and the density of photons increases: the number stays the same, but the volume decreases (density = mass/volume). But in addition, each photon carried more energy in past since the wavelength becomes smaller as we go back in time to when space was more compressed. This means that going back, the radiation becomes more dense faster than the matter, and ultimately we go back to a time when radiation dominated the universe. The cross-over point was at a few thousand years after the Big Bang. Before then, universe is mainly a bath of hot photons.

(Dark energy was presumably less important in the early universe than now.)

There were particles (protons, neutrons, electrons etc.) present in the early universe, but they were continually appearing and disappearing.

Pair creation reactions and photon annihilation reactions can transform particles into photons and vice versa. The pair creation reactions are the source of all particles now in the universe.

e+ e

e+ e

e+ e → → e+ e- At high Temp, the two reactions are in equilibrium: they occur at same rate and number of electrons and photons remains the same – until the time when the temperature drops and the energy of the photons is not enough to create the mass associated with the electron and positron. Occurs at a few billion degrees. Population of electrons declines drastically.

Similar creation and annihilation reactions for proton/antiproton pairs and neutron/antineutron pairs. At a temperature of about 1013 K, these pair creation reactions become impossible, and most protons/neutrons disappear through annihilation.

Passing these temperature thresholds is called ‘Freeze out’.

destroy electronscreate electrons

We know a great deal about the particle interactions from our experiments at accelerators, so this aspect of understanding the early universe is well understood. This is a particle collision seen in Stony Brook’s D0 experiment.

The accelerator at Fermi National Laboratory is a four mile circumference race track for protons and antiprotons.

2 km

Four fundamental forces among the particles:

In order of increasing size of the forces:

Gravity : all particles feel this, in proportion to their mass

Weak nuclear force : the force that operates in nuclear decay reactions like 15O → 15N + positron + neutrino that operates in the sun to fuse hydrogen into helium. Most particles feel this.

Electromagnetic force : force between charges: like signs repel causing it to be hard to get two nuclei together to react in the center of a star. Only charged particles and photons participate.

Strong nuclear force : causes reactions like 12C + p → 13N + energy in stars.

A host of particles experience these forces in different combinations – quarks (which make protons and neutrons), electrons, photons etc.

The 4 forces in our laboratory experiments are quite distinct and have very different strengths. But at very high energy or Temperature, the Strong, Electromagnetic and Weak forces are known to fuse into one and share common properties. We suspect that gravity may also merge with the other three at even higher Temperatures.

As the universe cools (energy decreases), one by one, the forces drop out of the unified whole. Called a phase change – like water freezing. Energy is liberated when the phase transition occurs.

A brief history of the universe: (times, density and Temp. are for end of epochs)

Epoch time density Temp character

Planck 10-43 s 1095 kg/m3 1032 K all 4 forces unified, vigorous particle production

Grand unification 10-35 s 1075 1027 only strong, electromag, weak unified Inflation occurs at the end of epoch

Hadron 10-4 s 1016 1012 all particles in equilibrium with photons

Lepton 102 s 104 109 only electrons/photons in equil.

Nuclear 103 yr 10-13 6x104 form helium and deuterium

transition between radiation dominated and matter dominated

Atomic 106 yr 10-19 103 atoms form at 380,000 yrs and CMB is born; clouds of atoms in universe

Galactic 109 yr 10-25 10 galaxies and clusters form

Stellar present 2x10-27 2.7 complex galaxies with bright stars and planets evolving; some intelligent (!) life

In the Planck epoch, gravity, weak, electromagnetic and strong forces are presumed to be unified (we don’t understand just how because we don’t have a theory of quantum gravity yet). All particles known (and others to heavy to be seen yet) were present in equilibrium with the very hot bath of photons. Universe extremely tiny, hot and dense.

In the Grand unification epoch, gravity drops out of unification, leaving unified strong nuclear, electromagnetic and weak nuclear forces unified. Near the end of this epoch, when T drops to 1028 K, the strong nuclear force also goes out of unification. At this point, some of the heavy particles associated with the combined unified forces ‘freeze out’ of the equilibrium and propagate freely. One of these particles (‘supersymmetric’ partners to the photon) is one of the best candidates for the dark matter, seen to fill the universe today.

At the end of the Grand unification epoch when the universe was around 10-35 seconds old, a mysterious phenomenon called Inflation seems to have occurred.

What happened in each successive epoch?

Inflation:Going from a universe in which strong, EM and weak forces are unified to one where strong differs from EM & weak is a phase transition. A familiar phase transition is water freezing to ice. It is possible in water be supercooled and to exist as liquid even below the usual freezing temperature. When the supercooled water finally turns into ice, it releases a great deal of energy suddenly. (A similar phenomenon occurs in reverse when you heat water very gently to above its boiling point – eventually any little speck of dust will initiate vigorous boiling.)

The universe seems to have supercooled in the symmetric state of all 3 forces unified for longer than it should have. When it ‘popped’ into the broken symmetry phase, a tremendous burst of energy was emitted that inflated the universe extremely rapidly.

Inflation lasted from about 10-35 to 10-32 seconds

Radius of universe expanded from 10-36 times the size of a proton to the size of a grapefruit – by a factor of more than 1050 !

At 10-32 s, after the energy was dissipated, the evolution of the universe resumed its more normal pace and the grand unification of forces was gone, leaving the separate strong and combined EM & weak forces.

Not necessarily all the universe at the time inflated, but all that is visible to us did so. We may be in one bubble among many.

Inflation blew up the universe’s size by a factor of 1050.

Inflation solves the flatness problem – just as if you blew a balloon up enormously, the previous spherical surface would look flat. And flat means that ≈ 1 (so the universe has critical density).

It also solves the horizon problem – two points on opposite sides of visible universe were close and in contact before inflation, and lost touch only after it.

By the end of the Hadron epoch at 10-4 sec, it became impossible for the reaction → p p to occur, since not enough energy was available to the photons to make the mass energy (E=mc2) of the proton and antiproton. This meant that the protons mostly disappeared, leaving only a very tiny remnant density that escaped before annihilating with antiprotons.

In the universe now, we see matter (protons, neutrons) and not antimatter (antiprotons, antineutrons). The laws of physics seem to require equal densities of each. We guess that some asymmetry in the laws of physics in the universe during the hadron epoch caused this, but it remains a puzzle how this works.

Until about 100 seconds, the electron and anti-electron (positron) remained in equilibrium with the photon bath. At the end of the Lepton epoch, the photon energies were too small to make e+ e, and electrons mostly disappeared leaving a tiny remnant population.

The small fraction of remaining electrons and protons after their freezeouts remain today, and determine the ratio of matter to photons in the universe. Cosmology can calculate this ratio, so it is a good check on the validity of our Big Bang model.

The protons and neutrons collide in the reaction p + n → 2H (deuteron =d ) + . But at high temperatures, the reverse reaction + d → p + n occurs as rapidly and keeps the deuteron density low. In the Nuclear epoch, when the temperature drops below a billion degrees, the reverse reaction stops (not enough energy to make it occur), and free deuterons appear.

Rapid formation of helium then occurs by combining deuterons, protons and neutrons – for example d + d → 4He + energy. About 25% of the matter is helium nuclei; about 75% is protons (hydrogen nucleus), and only about 0.01% remains as deuterons. The electrons are still too energetic to be captured by the nuclei.The helium and deuterium formed in the Nuclear epoch remain today – the helium in a balloon was made in the first few minutes of the universe (plus a little made later in burning in stars.)

The deuterium abundance depends on the total density of matter – the more matter present, the less deuterium remaining. The observed density of deuterium confirms that about 4% of the universe is composed of ordinary atoms.

The universe has now passed into its matter dominated phase. In the Atomic epoch, the temperature drops to the point that electrons can be captured by nuclei to form neutral atoms. Up until this point, the hot gas of photons interacted strongly with electrons, so was in equilibrium with the matter. From this ‘decoupling of radiation’ point onward, the photons were free to roam without interaction. We see them, red shifted by a factor of 1000, as the Cosmic Microwave Background.

In the Galactic epoch, the small fluctuations in density seen in the CMB pattern, reflecting also the matter fluctuations, seed the formation of clumps of gas from which the galaxies form. Actually the visible matter fluctuations revealed in the CMB pattern is insufficient to make the galaxies seen after about 1 billion years – but the dark matter clumping in the early universe could account for the early galactic formation.

Simulations of gravitationally induced structure using visible matter (atoms) and dark matter agree with the structure of galaxies and galactic clusters that we see. The dark matter is a necessary ingredient !

Temperature fluctuations from decoupling

Matter clumping due to fluctuations

First stars at 200 million years

Galaxy forming along lines of density fluctuations in frame 2

Modern era

The evolution of structure:

cosmic microwave background to clumps of matter to stars and protogalaxies to our present day universe.

Powers of 10 in distance – a 10 minute tour of the universe to the very large and to the very small.

Lets see in 10 minutes the whole span of this course.

1 m

The picture of the sleeping picknicker in Golden Gate Park in San Francisco is 1 meter square. The small blue square is 0.1m =10 cm on each side.

Successive pictures will enlarge the frame by a factor of 10. The current outer frame will be shown as the small frame in the next picture.

10 m

On the grass at the park

102 m

The Music Concourse at Golden Gate Park

103 m = 1 km

Golden Gate Park and its roads

104 m = 10 km

San Francisco

105 m = 100 km

San Francisco Bay area

106 m = 1000 km

California

107 m = 10,000 km

North America

108 m = 100,000 km

Earth

109 m = 1 million km

Earth and Moon’s orbit

1010 m

Four days on Earth’s orbit

1011 m

Venus, Earth and Mars orbits

Earth

Venus

Mars

1012 m

Planetary orbits out to Jupiter

Jupiter

1013 m

The solar system

1014 m

Our sun and its little solar system

1015 m

Sol – our lonely sun

1016 m ≈ 1 light year

The Oort cloud – birthplace of comets

1017 m ≈ 10 light years

The sun and the nearest stars

Alpha Centauri

1018 m ≈ 100 light years

Nearby stars in the galaxy spiral arm

1019 m ≈ 1000 light years

The stars of the Orion spiral arm of the galaxy

1020 m ≈ 10,000 light years

The full spiral arm

1021 m ≈ 100,000 light years

The Milky Way

1022 m ≈ 1 million light years

The local group

1023 m ≈ 10 million light years

Within the Virgo supercluster

1024 m ≈ 100 million light years

Clusters of galaxies

1025 m ≈ billion light years

Large scale structure in the universe

1 m

The picture of the sleeping picknicker in Golden Gate Park in San Francisco is 1 meter square. The small blue square is 0.1m =10 cm on each side.

Succeeding pictures will magnify the small square to the full frame.

10-1 m = 10 cm

A hand

10-2 m = 1 cm

Skin

10-3 m = 1 mm

A pore on the skin

10-4 m = 100 microns

Micro-organisms

10-5 m = 10 microns

A lymphocyte

10-6 m = 1 micron

Nucleus of a cell

10-7 m = 100 nanometers

Strands of DNA

10-8 m = 10 nanometers

The structure of DNA

10-9 m = 1 nanometer

Molecules of DNA

10-10 m = 0.1 nanometer = 1 angstrom

Carbon atom’s outer shell of electrons

10-11 m = 10 picometers

The inner cloud of electrons in carbon

10-12 m = 1 picometer

Inside the electron cloud –the nucleus is just visible

10-13 m = 100 femtometers

A carbon nucleus

10-14 m = 10 femtometers

Carbon nucleus up close – six protons and six neutrons

10-15 m = 1 femtometer

Inside the proton – a swarm of quarks

10-16 m = 100 attometer

The quarks

The laws of physics at the level of quarks and photons control the structure and evolution of the universe at large.

Inner space and outer space are intertwined.

Element formation in stars

The big bangDark matter

shapes the galaxies

Forming Earth-like planets

Chemistry of life

Fundamental particle forces shape the future universe

Element formation in stars

Laboratory particle physics and cosmology let us look back to the big bang