© sierra college astronomy department cosmology ---------------- the beginning of time

© Sierra College Astronomy Department

Cosmology----------------

The Beginning of Time

© Sierra College Astronomy Department 2

Cosmology: The Beginning of Time

The Big Bang

• Hints of a Beginning– Up until the early 20th century, the origin of the Universe

was mostly covered by the domains of philosophy and theology.

– Then Einstein’s General Theory of Relativity gave science a framework to discuss the Universe’s origin.

– Observations also mounted that gave credence to a scientific investigation into origins:

• An expanding Universe seen in the redshifts of galaxy clusters

• Change in galaxies as a function of distance (time)• Observations of the H/He ratio• The cosmic microwave background

– Observations and theory lead us back to the time of the Big Bang – a time that, as yet, can only be theoretically described.



The Big Bang• An Expanding Universe in Reverse

– Traveling back in time before the formation of the first stars and galaxies, the Universe is filled with particles of matter and radiation.

– As the size of the Universe continues to decrease, the temperature and density of matter and radiation increase.

– Eventually, we reach a time of such extreme conditions that particles of matter and radiation are continuously transforming into other particles of matter and radiation.

• In particular, matter and anti-matter pair annihilation forms gamma-ray photon pairs.

• The reverse process of matter and anti-matter pair creation from gamma-ray photon pairs also occurs.

– Theoretical physics can take us back, with reasonable confidence, to an age of the Universe of about 10-10 second; earlier times require “missing physics”.

– The “soup of matter and radiation particles” can be broken into seven eras.



The Seven Eras of the Early Universe

• The Planck Era– Time: Less than about 10-43 second– Physics of this time period is unknown.

• Quantum Mechanics is a highly successful theory of the very small.• General Relativity is a highly successful theory of gravity (mass) and

the very large.• A theory of very large mass on small scales does not yet exist

(Superstring Theory?)

– Planck Era is expected to experience very large energy fluctuations (from Heisenberg Uncertainly Principle).

• From E = mc2, these large energy fluctuations translate into large gravity (mass) fluctuations.

• Large gravity fluctuations mean large fluctuations in curved spacetime structure.

• Quantum Mechanics is based on a flat spacetime – hence the problem with using our current physics in the Planck Era.




• The GUT Era– During the Planck Era, it is believed that the four distinct forces

of today (gravitational, electromagnetic, strong, and weak) were indistinguishable and essentially explained by one force law.

– The beginning of the GUT Era (at 10-43 second) is marked by the splitting of gravity from the other three forces, which are described as one force (the GUT force) by Grand Unified Theories (hence the acronym GUT).

– At the end of the GUT Era, the strong force separates from the electroweak force.

• This occurs at a temperature of about 1029 K when the Universe is 10-38 second old.

• The freezing out of the strong force releases a tremendous amount of energy which drives inflation, which lasts about 10-36 second and increases the size of the Universe from that of an atom to that of the Solar System.




• The Electroweak Era– This era lasts until the temperature has decreased to 1015

K when the Universe’s age was 10-10 second.– At the end of the Electroweak Era, all four forces are now

distinct.– The temperature at the end of the Electroweak Era is

about 100 million times hotter than the center of the Sun.• Experiments have been conducted at these energies

– Detected the electroweak W and Z (or weak) bosons, carriers of the electroweak force

– This detection confirms the electroweak theory, at least for temperatures as high as 1015 K.

• These successful experiments are what give scientists confidence about predicting the state of the Universe from 10-10 second to today.



The Seven Eras of the Early Universe• The Particle Era

– At the beginning of the Particle Era, spontaneous creation/annihilation keep the number of photons and particles roughly in balance.

– As the temperature of the Universe lowers, less and less photons have sufficient energy to create particles.

– The photon-to-particle ratio increases with time• The variety of particles settles into the more stable forms: protons,

neutrons, electrons, neutrinos, and perhaps WIMPs.• A slight imbalance of matter over antimatter results in a Universe of

matter rather than antimatter.– The current ratio of photons-to-protons is about 109:1– Consequently, for each 1 billion protons and antiprotons that

annihilated each other, one proton was left over at the end of the Particle Era.

– The Particle Era ended at 0.001 second and a temperature of about 1012 K.

• Some of the protons and neutrons from this point will eventually make their way in the bodies of humans.



The Seven Eras of the Early Universe• The Era of Nucleosynthesis

– The Nucleosynthesis Era starts at 0.001 second (faster than a blink of an eye) and lasts until the Universe is about 3 minutes old.

– The temperature drops from 1012 to 109 K.– This period is noted for fusion and breakup of nuclei containing

protons and neutrons.– The end of the Nucleosynthesis Era

• Fusion ceases even though temperature is higher than center of Sun (density is now too low)

• 75% of the ordinary (baryonic) mass in the Universe is hydrogen (a proton), 25% is helium, and a trace percentage is deuterium and lithium.

• Except for small percentage heavier elements forged in stars, composition of the Universe will not change much after the end of the Nucleosynthesis Era.

• The 75%/25% ratio is in fact observed today.



The Seven Eras of the Early Universe• The Era of Nuclei

– At the beginning of this era, the Universe is a hot plasma of hydrogen, helium, and free electrons.

• This basic picture stays the same for 370,000 years as the Universe grows and cools.

• Matter and photons are coupled (collide frequently, changing their directions of travel).

• Any ionized hydrogen or helium that acquire an electron are quickly ionized again.

– At the end of the Era of Nuclei, the temperature reaches 3,000 K.

• Hydrogen begins to capture electrons to form neutral atoms that will remain so for very long periods of time.

• The Universe becomes transparent as photons are now free to pass by nuclei without being absorbed (the photons are no longer energetic enough to lift electrons to higher atomic orbits).

• These free-flowing photons are now seen as the cosmic microwave background.




• The Era of Atoms– Once the Era of Atoms begins, the formation of

neutral atoms is in full swing.– Matter begins to clump around concentrations of

dark matter.– As the clumping becomes significant,

protogalactic clouds begin to emerge and the Era of Galaxies begins with an age for the Universe standing at about 1 billion years.

• We are still in the Era of Galaxies – some 14 billion years after it all started.



Cosmic Microwave Background

• Some History– Steady-State Universe model competed with Big

Bang model until 1960s.– In 1965, Arno Penzias and Robert Wilson of

Bell Laboratories in New Jersey accidentally discovered cosmic microwave background (CMB)

• CMB first predicted in 1940s by George Gamow, but ignored.

• Predicted again by Princeton group at about the same time as Penzias/Wilson discovery.

• Penzias and Wilson receive Nobel Prize in 1978.



Cosmic Microwave Background• Basic Physics

– CMB is believed to come from the decoupling of photons from matter at the end of the Era of Nuclei. Theory shows:

• Age of Universe: 380,000 years• Temperature: 3,000 K

– As measured from COBE and WMAP spacecraft, the CMB today is 2.73 K and a near perfect thermal spectrum.

max = (2,900,000/2.73) nm = 1.06 mm (microwaves)– Projecting back in time:

max (at T=3000 K) = (2,900,000/3000) nm = 970 nm (IR)• Universe has expanded by factor 1 + z = 1.06x106/970 =

1,100



Cosmic Microwave Background• Details of CMB

– CMB measured across the sky shows variations of a few parts in 100,000 (with motion of spacecraft relative to CMB removed)

– These small variations are believed to reflect increased ordinary baryonic matter densities - the centers upon which galaxies will eventually form.

– However, galaxies started forming within about 1 billion years of Universe creation.

• Implied baryonic densities from CMB enhancements not enough to create galaxies that fast.

• WIMPs, since they do not interact with photons and will not enhance CMB, must have formed much higher density concentrations before baryons.

• WIMP concentrations allow galaxies to form faster.• CMB enhancements then must be echoing WIMP density.



Element Abundances• Basic Physics

– The existence of the CMB invites us to compress the Universe to a point that the temperature reaches 1012 K.

• This is the start of the Era of Nucleosynthesis.• Our best theories show that protons and neutrons undergo

annihilation/creation and are about equal in number.– As the Universe cools to 1010 K (3 minutes after the Big

Bang), proton creation is favored and neutron creation ceases.

• Neutrons are more massive and not as stable as protons.• The proton:neutron number ratio reaches a value of 7:1.• Fusion reactions lock neutrons into some deuterium, but

mainly helium-4.• The 7:1 ratio translates into a 75%/25% H/He ratio by mass,

which is also observed.



Element Abundances

• Details of Element Abundance– By the time helium is fused, the Universe is expanding and

cooling too fast to fuse anything more massive than lithium-7 (which has 3 protons and 4 neutrons).

– Calculating the abundances of deuterium and lithium as a function of critical density, it is found that to match observations, ordinary baryonic matter must make up 4% of the critical density.

– Since other measurements show the density of the Universe is close to 25% of critical, the remainder must be dark matter.

– The amount of dark matter to ordinary matter then is 6 times larger and most likely WIMPS (again).



Big Bang Problems• Basic Problems with Standard Big Bang

– Where does the structure come from?• Galaxies are thought to be seeded by the mass density variations

implied the variations in the CMB.• These CMB variations in turn imply dark matter variations.• But where do these dark matter variations come from?

– Why is the large-scale Universe so uniform (this is known as the Horizon Problem)?

• The CMB implies that the density of the Universe at the end of the Era of Nuclei was no more than 0.01%.

• This is a remarkable smoothness considering the vast expanse of the Universe, even back then.

– Why is the total density of matter and dark energy so close to the critical density (this is known as the Flatness Problem)?

• Can it just be a coincidence?• Or is there some underlying reason?



The Inflation Model

• The Inflation Addition to the Standard Big Bang– In 1981, Alan Guth proposed that Grand Unified

Theories could solve the problems of the Standard Big Bang.

• When the GUT force separates into the strong and electroweak forces, a tremendous amount of energy is released.

• The expansion factor is 1030 in about 10-36 second and occurs right after the Big Bang itself!!

• This sudden and tremendous expansion is called inflation.



The Inflation Model

• Inflation and CMB Variations– Quantum Mechanics and the Uncertainty Principle state

that at any point in space there are random energy fluctuations – even in a “vacuum”.

– These energy fluctuations can be enormous at the beginning of the Universe, but they are still microscopic in physical extent (small wavelengths).

– Inflation increases the physical size of these energy (and mass) fluctuations to Solar System sizes by 10-36 second after the Big Bang.

– Consequently, the variations seen in the CMB are believed to be the result of the quantum fluctuations at the very beginning of time.



The Inflation Model• Inflation and the Horizon Problem

– Other than being a coincidence, for any two regions in space to have identical properties:

• They must be able to “communicate” with each other by physical contact or the transmission of information.

– For every direction of the CMB to have a near identical temperature, it cannot be a coincidence.

– However, the Standard Big Bang with its nearly linear expansion rate, will not have allowed vastly distant regions today to have communicated any time in the past.

– Inflation overcomes this obstacle by having any two regions within at least our current cosmic horizon being in contact before the onset of inflation.

– Note: Inflation does not violate the speed-of-light limitation since objects in the inflationary era are moving apart due to the expansion of space and not by their motion through it.



The Inflation Model• Inflation and the Flatness Problem

– General Relativity predicts three categories of shape for the Universe

• Flat (or critical) with the spatial geometry that of Euclid• Spherical (or closed) with the spatial geometry similar to that found

on the surface of a sphere.• Saddle Shaped (or open) with the spatial geometry similar to that

found on the surface of a saddle.– Important Consequences

• Which shape the Universe takes depends on the density of the Universe relative to its critical density.

• If at the end of the Era of Nuclei the Universe deviated just a few percent from critical, the Universe would have collapsed or expanded too fast to create the Universe we see today.

– Inflation’s enormous expansion rate forces the Universe to appear flat within the observable Universe.

• This solves the flatness problem• Dark energy needed to bring the density up to critical



The Inflation Model

• More Evidence of Inflation– Using quantum mechanics to calculate the distribution in

sizes of quantum fluctuations, the size distribution of the CMB temperature fluctuations can be determined for a flat Universe.

– The largest temperature fluctuations are predicted to be about 1° apart (larger if the Universe were closed and less if the Universe were open).

• Measured angular separations of the largest temperature fluctuations are found to be about 1° apart!

• Predictions and observations also match for other relative sizes of temperature fluctuations and their sparations.



The Inflation Model• The temperature fluctuation distribution data can

also be used to determine the a number of properties of the Universe– Overall geometry is flat.– Ordinary (baryonic) matter is ~5% of the critical density (in

agreement with deuterium observations).– Total matter density is 28-31% of critical density leaving

about 23-26% in the form of extraordinary (non-baryonic) dark matter (in agreement with galaxy cluster measurements).

– 72-69% of the Universe’s mass-energy must be in the form of dark energy (in agreement with observed accelerated expansion of the Universe).

– The age of the Universe must be about 13.8 billion years (in agreement with what we infer from Hubble’s constant and the ages of the oldest stars).



Olbers’ Paradox• A Paradox or Not

– Olbers’ paradox is an argument showing that the sky in a static and infinite (in age and space) universe could not be dark.

– If stars and galaxies are distributed throughout all infinite space, then in whatever direction one looked, one should see a star.

– However, the night sky is in fact primarily dark and we see relatively few stars – hence the paradox.

• The Solution is:– The solution to Olbers’ paradox – the reason the sky is dark –

starts with the age of the universe being finite (the Big Bang Theory).

– As we look outward into space we are looking back in time. Consequently, we can only observe a finite amount of stars.

– The expansion of the Universe, light obstruction by dust, and other ideas have been also suggested as solutions, but all these possibilities are limited in explaining the darkness.

© sierra college astronomy department cosmology ---------------- the beginning of time

Documents

beginning of time slide

planck era time

particles of matter

time period

large fluctuations

gut era

early universe

large gravity fluctuations