Science is Seeing

Space

A Scale Model

Set the Earth to the size of a walnut, or a 1:10,000,000 scale model

=

• The Moon is a pea at arm’s length • The Sun is a 3 m ball 100 m away • Neptune is another pea 2 km away • The nearest star is 50,000 km away

• The Moon is a seconds walk away • The Sun is 8 minutes walk away • 10 hours to walk the Solar System • A year to walk to the nearest stars

And at this scale, light is reduced to very slow walking speed. There’s no way information in the universe can travel any faster

Reduce the scale by a factor of 100,000,000

• The Solar System is a grain of sand • The distance between stars is 10 m • The Milky Way is the size of the U.S. • The MW has 100,000,000,000 stars

Now reduce by another factor of 100,000,000

• The Milky Way is the size of a plate • The nearest galaxy is 10 m away • The universe is the size of the U.S. • Billions of galaxies within this space

How Empty?

A one-inch cube of the air you’re breathing holds 1020 atoms in it

The average density of the universe is 1022 times lower, about 1 atom per cubic meter

For an idea of the true density of the universe, imagine the 1-inch cube of air stretched upwards

…all the way to the Andomeda galaxy (M31)

Olbers Paradox

“The night sky is dark.” This statement is Olbers’ paradox, after astronomer who discussed the subject in 1823, before galaxies were discovered.

Why is the darkness of the night sky paradoxical?

In an infinite universe with infinite number of stars, a mathematical paradox arises.

In a large enough forest, every line of sight ends at a tree. In a large enough universe, every sight line ends in a star. The sky should not be dark.

The surface area of a spherical shell:

Area = 4 π r2

The volume of the spherical shell:

Volume ≈ area × thickness ≈ 4 π r2 t

How many stars are in the shell?

Number = volume × n = 4 π r2 t n

The flux from a single star: 2r 4

L Flux

The flux from all the shell’s stars:

Total flux = Number of stars × flux per star

2

2

r 4

L n t r 4 flux Total

Total flux of shell = t × n × L

Total amount of light is independent of r, the distance of the shell from us!

Total flux from shell = t × n × L

# of stars per cubic parsec luminosity of single star

Number of stars goes up at the same rate as the light from each goes down.

1. The expanding universe – light from distant objects is reduced in energy by the cosmological redshift.

2. The finite age of the universe – light from distant galaxies has not had time to fill the voids of space.

The Paradox Explained:

Old Light

Lookback Time

If the speed of light were infinite, light from everywhere in the universe would reach us at exactly the same time and we would see the entire universe as it is now.

But it is not, so we see distant regions as they were in the past.

How can we know what the universe was like in the past?

• Light travels at a finite speed (300,000 km/s).

• Thus, we see objects as they were in the past:

The farther away we look in distance,

the further back we look in time.

Destination Light travel time

Moon 1 second

Sun 8 minutes

Sirius 8 years

Andromeda (M31) 2.5 million years

We know of an ancient radiation That haunts dismembered constellations, A faintly glimmering radio station. While Frank Sinatra sings Stormy Weather, The flies and spiders get along together, Cobwebs fall on an old skipping record. Beyond the suns that guard this roof, Beyond your flowers of flaming truths, Beyond your latest ad campaigns, An old man sits collecting stamps In a room all filled with Chinese lamps. He saves what others throw away. He says that he'll be rich some day. We know of an ancient radiation That haunts dismembered constellations, A faintly glimmering radio station.

It’s 50 years since the first SETI transmissions. But the outer shell of our accidental radio messages, containing early work of Frank Sinatra, has swept over the planets around thousands of stars.

Example:

This image shows the Milky Ways’ neighbor, the Andromeda

Galaxy (or M31) as it looked about 2.5 million years ago. Question: When will be able to see what it looks like now?

M31, Andromeda

This galaxy is 100 million light years away. We see it not as it is now but as it was 100 million years ago. Distant light is old.

Measurements of this star show that is it 10 million years old. It is a type of star that lives a total of 50 million years and dies as a supernova.

Let’s see how the light and information actually travel

100 million light years

• It has taken 100 million years for the galaxy light to reach us. As we see it, the bright star within the galaxy is 10 million years old and will have a total lifetime of 50 million years.

Has the supernova event already occurred and if so, when?

Nearby “Now”

“Long Ago” Galaxies

LOOKBACK TIME

“Recent” Stars

“Ancient” Universe

The Contents

Scattered in a universe 46 billion light years across

Galaxies

The Milky Way is typical with 400 billion stars

Stars

Almost all the simple elements hydrogen and helium

Atoms

1080

A billion photons for every particle

Photons

1089

Mediocrity

We therefore live on an: • Average planet around • An average star in an • Average galaxy in a • Very large universe

Copernicus

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Composition of the Universe

Dark matter: 22%

Dark energy: 73%

Universal Pie

Us

WE’RE NOT SPECIAL

Universe

Cosmology

Cosmological Principle

Homogeneous means the same, on average, at any location. This wall extended is homogeneous but it’s not the same in any direction.

Isotropic means the same, on average, in any direction. This pattern is isotropic as viewed from the center but it’s not homogeneous.

Cosmological Principle

Cosmology involves the assumption that the universe is homogeneous on large scales and isotropic. Isotropy is verified by galaxy surveys in different directions, but the assumption of Homogeneity can’t be tested because we are forced to look back in time as we look out in space.

Laws of Physics

There’s also an assumption that the laws of physics are invariant across time and space, which is very difficult to test experimentally.

An even deeper assumption is the rationality of nature, that there are laws of physics, and that causality holds.

Testing for Variation

One recent experiment looked for subtle changes in atomic transitions between light in the lab and light from a distant quasars. The physical constant at issue is the fine structure constant. Change was seen but the significance level was low.

Cosmology: What We Know

1. Redshift – it’s a cosmic

expansion of space-time,

not a Doppler shift.

2. The microwave background radiation (CMB), a signal that is close to perfectly thermal, at temp of 2.726K.

3C 273

© 2005 Pearson Education Inc., publishing as Addison-Wesley

3. Deuterium and Helium synthesized (much higher temperatures in the past).

Hubble Ultra Deep Field HST•ACS

4. Galaxies in past look younger (smaller and more irregular).

Cosmological Parameters

• A variety of observations affirm the presence of dark matter on large scales, exceeding baryons.

• Supernovae show accelerating expansion, attributed to dark energy, i.e. repulsive vacuum.

• CMB observations show that space curvature is small and confirm the age of 13.7 Gyrs.

• These observations all agree with a model where cosmic expansion continues forever.

Microwave Background

Cosmic Microwave Background

• The universe is immersed in a sea of radiation.

• 380,000 years after the big bang, the universe had cooled enough for free electrons to become bound into atoms of Hydrogen and Helium

• Without electrons to scatter them, photons were able to travel unhindered throughout the universe

• The universe became transparent

• The temperature of the universe was 3,000 K at this time, similar to a stellar photosphere

• It has expanded by a factor of 1000 since then, reducing the temperature to 3000/1000 = 3 Kelvin

The Infant Universe

A Thousand Times Smaller

The CMB photons have been stretched by the cosmological redshift, and have the highest redshift of anything we can observe (z = 1000).

When we look at the CMB, we look at the surface of the glowing “fog” that filled the entire early universe!

The Big Bang is Everywhere

There is evidence for expansion, and the universe was hotter and denser in the distant past. The microwave background has been shown to be a signal at cosmological distance, not a local signal. Hundred of thousands of big bang photons permeate every breath you take: the big bang is everywhere. The big bang theory naturally explains the microwaves, which are very hard to explain any other way.

Microwave Background

1% of the specks

on any TV tuned

between stations

are interactions

with the big bang

1% of the specks

on any TV tuned

between stations

are interactions

with the big bang

Big Bang

• A good scientific model should always make predictions which can be verified.

• The big bang model makes two predictions which have been verified since the 1960s:

• the existence and characteristics of the cosmic microwave background (CMB)

• the expected Helium abundance in the Universe

• The model predictions agree with all current observations. There is much indirect evidence the universe was smaller and hotter in the past

Evidence

Space is Flat

The Flatness Problem

For the matter density now to be so close to critical, the boundary between eternal expansion and a subsequent collapse, requires extraordinary fine-tuning in the early universe on a special value.

For values of the matter density much different from the present value, the universe would either have collapsed and lived a much shorter time or expanded rapidly enough for no stars and galaxies .

Horizons

The Earth has a horizon: we can’t see past it because of the Earth’s curved surface.

A black hole has an event horizon: we can’t see into it because photons can’t escape.

The universe has a cosmological horizon: beyond it photons haven’t had time to reach us in 13.7 billion years since the BB.

Light travels simply and directly in the local universe..

But early expansion of the universe carried any two points away from each other faster than light speed. Distant galaxies are only just becoming visible now.

PHYSICAL UNIVERSE >> OBSERVABLE UNIVERSE!

The Smoothness Problem

What COBE and other satellites saw was an extremely smooth radiation, with the temperature not varying by more than 1 part in 1000 in any direction in space.

Heat or energy must travel for temperatures to equalize. The radiation from the universe is much to uniform to understand because patches a degree apart were expanding faster than light.

Cosmic Inflation

The standard big bang has trouble explaining why the universe is as smooth and flat as it is, leading to the idea of an epoch of extremely rapid inflation, just 10-35 sec after the big bang. The mechanism is unclear but probably associated with the Grand Unified theories that seek to unite all the forces except gravity.

Status of Inflation

Inflation makes the universe flat and smooth (by design!), and it implies vast region of space that are beyond view. It has tentative support from CMB satellite data.

Combining CMB with other data, a key characteristic of inflation is observed, deviation from pure power law of equal power on all spatial scales (a tilted power spectrum). The other key prediction involves the strength of gravity waves.

© 2005 Pearson Education Inc., publishing as Addison-Wesley

There is evidence for expansion, and

the universe was hotter and denser in

the distant past.

The microwave background and the

helium abundance cannot easily be

explained in any other way.

There are hundred of thousands of

big bang photons in every breath you

take: the big bang is all around us.

It is a theory, but a theory with a web

of evidence to support it. The theory

is mute about the cause of the cause.

Status of the Big Bang

Early Universe

History of the Universe

Planck Era (t < 10–43

sec) • This era, the “first instant”, lasted for 10

–43 sec.

• Because we are as yet unable to link…

• quantum mechanics (our theory of the very small)

• general relativity (our theory of the very large)

• We are powerless to describe what happened in this era.

• 10–43

sec after the big bang is as far back as our current science will allow us to go.

The big bang was extraordinary― the instantaneous creation of all of space and time, containing energy to drive the expansion and enough matter for 100 billion galaxies. The initial state was so compact that it can only be described by a theory that unites gravity and the quantum world. We do not have such a theory at present. The big bang can be thought of as a quantum event, originating from very chaotic space-time in which the other quantum fluctuations might have led to other parallel universes.

An image of quantum

fluctuations blown up to

the size of the universe

LHC

10-12 s

Particle Era (t < 0.001 sec; T > 1012 K)

• Particles were as numerous as photons.

• At 10–6 sec old, the universe cooled to 1012 K.

• p+, p, n, and n could no longer be created from two photons

• the remaining particles and antiparticles annihilated each other into pure radiation

• slight imbalance in number of p and n allowed a small residue of matter to remain

• e and e+ are still being created from photons.

– until t = 4 sec; T > 5 x 109 K

Nucleosynthesis Era (t < 3 min; T > 109 K)

• During this era, p+ and n started fusing…

• but new nuclei were also torn apart by the high temperatures

• When the universe was 5 minutes old…

• it had cooled to 1010 K and the fusion stopped

• Afterwards, the baryonic matter leftover in the universe was:

• 75% Hydrogen nuclei (i.e. individual protons)

• 25% Helium nuclei

• trace amounts of Deuterium (H isotope), Helium-3, and Lithium nuclei

Cosmic Helium Abundance • In the Era of Nucleosynthesis

• number of p+ and n roughly equal as long as T > 1011 K

• below 1011 K, proton-to-neutron reactions no longer occur • neutrons still decay into protons

• protons begin to outnumber neutrons

• At T < 1010 K, He, Deuterium, and Li remain stable

– At this time, big bang model predicts a 7-to-1 p+-to-n ratio.

• For every 2 n and 2 p+ which fused into a Helium nucleus… • there are 12 p+

• Model predicts a 3-to-1 H:He

• This what we observe: • minimum of 25% He in all galaxies

Era of Nuclei (3 min < t < 380,000 yr)

• The universe was a hot plasma of H & He nuclei and electrons.

• photons bounced from electron to electron, not traveling very far

• the universe was opaque (like inside a cloud)

• When the universe was 380,000 yrs old…

• it had cooled to a temperature of 3,000 K

• electrons combined with nuclei to form stable atoms of H and He

• the photons were free to stream across the universe

• the universe became transparent

Era of Atoms (380,000 < t < 109 yr)

• The universe was filled with atomic gas.

• referred to as the “Cosmic Dark Ages”

• Density enhancements in the gas and gravitational attraction by dark matter…

• eventually forming proto-galactic clouds

• the first star formation lights up the universe

• the first heavy elements are created

• closely followed by the formation of galaxies

Present and Future

Age of the Universe

The age in a linear expansion model is 1/H0. Dark matter causes deceleration then dark energy caused speeding up so the net result is still close to 1/H0. The age in a model is 13.7 Gyr, with an error of only 3-4%.

Age of the Universe

A cross-check comes from the age of the oldest star clusters in the halo or the oldest white dwarfs in the Milky Way disk.

Another cross-check is the abundance of the decay products of heavy isotopes. These two test are not very accurate but they verify the big bang model and age.

Era of Galaxies ( t > 109 yr)

• The first galaxies came into existence about 300 to 500 million years after the big bang.

• This is the current era of the universe, it is mostly composed of large, cold vacuum.

Infinite Universe?

The very early universe expanded so rapidly there are regions of space we’ve not seen.

Speed of Light

Size

Time

The Universe

The early universe expanded much faster than the speed of light, so there are objects and large regions of space we have never seen. Dark energy will remove more from view.

This violates no law of physics since the cosmic expansion is governed by general relativity, which sets no limit on the speed of expanding space.

Dark matter binds galaxies and dark energy is currently driving the cosmic acceleration.

Fate of the Universe

The early expansion was rapid, driven by intense radiation. It slowed down as the dark matter began to dominate, and more recently has begun to accelerate due to the relative growth of dark energy. With what we know now, it seems that it will expand forever.

Within the expanding and cooling universe, gravity forms stars and galaxies (and people)

Multiverse

Us

Milky Way

Earth

THE UNIVERSE AND US

Solar System

Universe Multiverse?

Science is Seeing

Life occurs in a range of scales

that extends from galaxies to the

atomic nucleus, as symbolized by the ancient symbol of the ouroboros, the snake that eats its

own tail

Cosmic Zoom

Beyond the Horizon The universe is bounded in

time and not space. General

relativity sets no speed limit

to the expansion. As time

goes by, ever more distant

regions come into view.

At z = 1.3, an object was moving

away from us at c at the time the

light was emitted.

At z = 1000, two distant points

were moving apart at ~60c at the

time the radiation was emitted.

Consequence of standard big bang: The physical universe is much

larger than the observable universe, we are subject to a horizon.

Consequence of the inflationary big bang: A microscopic region of

space-time became our universe; the universe is a quantum entity.

Quantum fluctuations are a mechanism

for multiple realizations of the universe

…leading to the concept of the “multiverse”

More than just this…

LEVEL 1: regions we can not see in big bang model LEVEL 2: many bubbles of space-time, unobservable by us, different properties LEVEL 3: indeterminacy, and quantum variation LEVEL 4: mathematical forms, multi-dimensional space-times, 10 preferred

String Theory Landscape

Perhaps 10 different vacua

500

de Sitter expansion in these vacua create quantum fluctuations and provide the initial conditions for inflation. String theory provides context for the “multiverse”

Landscape of eternal inflation

A. Linde

The Multiverse

1. The observed big bang is just one mediocre member of an (infinite) ensemble.

2. For a flat distribution of Pa

priori (rV), the requirement that galaxies can form gives rV that is quite consistent with the observations.

3. If the scalar field that generates the potential couples only to gravity anthropic ideas apply only to the cosmological constant.

Our universe emerged from a quantum space-time foam at the

Planck epoch. Other universes may have been spawned this

way, with randomly different physical properties. Most of the

universes would be inhospitable to life → anthropic principle.

Self-reproducing Inflationary Universe

“Whenever a creature was faced with several possible courses of action, it took them all, thereby creating many distinct histories of the cosmos. Since there were many creatures and each was constantly faced with many possible courses, and the combinations of all their courses were innumerable, an infinity of distinct universes exfoliated from every moment of every temporal sequence.” O. Stapledon 1937, Star Maker

The ekpyrotic universe has a big bang, but it is not ever at an infinite temperature and density, and it is not the origin of all space-time… So our universe emerged from a collision of two 4D branes that are embedded in 5D space-time. The collision is the engine for expansion and matter creation.

Eternal Universe