Dark Matter and Dark Energy

Dark Matter Dark Matter & Dark Energy& Dark Energy

Dr. Bryan J. Higgs28 November, 2012

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Dark Matter & Dark Energy

Over the past 35 years or so, cosmologists’ and physicists' understanding of the universe has been turned on its head.

It is now generally accepted in the scientific community that ‘normal matter’ — the matter that we experience in our everyday lives, and that scientists have been studying since the time of the ancient Greeks — comprises only about 4% of the matter in the universe.

So, what is the other 96%?

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Goals of the Talk

» To describe some of the evidence (and history) for why scientists believe that Dark Matter and Dark Energy exist.

» To describe what scientists have proposed to explain these observations.

» To describe the implications for the beginning and the end of the universe.

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History & Background

First, we need to look at some background on the history and observations that led us to this point.

Cast your minds back about 100 years...

(Yup, buggy whip time!)

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Einstein's Theory of General Relativity

In 1916, Albert Einstein published his Theory of General Relativity.

It provided a unified description of gravity as a geometric property of space and time.

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Assumptions

After the introduction of General Relativity a number of scientists, including Einstein, tried to apply the new theory to the universe as a whole.

This required an assumption about how the matter in the universe was distributed.

The simplest assumption to make is that if you viewed the contents of the universe with “sufficiently poor vision”, it would appear roughly the same everywhere and in every direction.

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The Cosmological Principle

That is, they assumed that the matter in the universe is:– Homogeneous

and

– Isotropic

when averaged over very large scales.

This is called the Cosmological Principle.

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The Static Universe Model

One hundred years ago, astronomers thought:The universe was unchanging through time.

The stars of our galaxy (the Milky Way) made up the whole universe

The galaxy was nearly motionless

Physicists trying to create a model for the universe had to match these "facts".

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Einstein's Theory of Gravity

Einstein created his model of the universe, based on his General Theory of Relativity, using these assumptions.

He came up with his famous Field Equations.

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Einstein's Field Equations

Note, in particular, the second term on the left, the one that includes the

famous Λ (Greek capital letter lambda)...

The Einstein Field Equations are a set of 10 equations in Albert Einstein's general theory of relativity which describe the fundamental interaction of gravitation as a result of space-time being curved by matter and energy.

The expression on the left of the = sign represents the curvature of space-time.

The expression on the right of the = sign represents the matter/energy content of space-time.

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The Cosmological Constant

Λ is the famous Cosmological Constant.

It is equivalent to an energy density in otherwise empty space (the vacuum).

It was originally proposed by Einstein as a modification of his original theory to achieve a stationary universe, to match what he thought was the known situation.

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The Fate of the Universe

There are many possible solutions to Einstein's Field Equations, and each solution implies a possible ultimate fate of the universe.

Alexander Friedman proposed a number of such solutions in 1922, as did the Belgian Jesuit priest Georges Lemaître in 1927.

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Fate depends on Density

Essentially, the various models of the evolution of the universe depend on whether or not there is enough mass in the universe to cause it, through gravitational attraction, to contract unto itself (the “Big Crunch”).

So how much mass is there in the universe?

How do we weigh the universe?

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The Density Parameter

The density parameter, Ω, is defined as the ratio of the actual (i.e. observed) mass density, ρ , of the universe to the critical density, ρcrit , of the universe.

To date, the critical density is estimated to be approximately five atoms (of hydrogen) per cubic meter. Not so much!

So what's the significance of thecritical density?

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The Shape of the Universe

The Friedmann–Lemaître–Robertson –Walker (FLRW) model has become the most accepted theoretical model of the universe. It is sometimes called the Standard Model of modern cosmology.

This model describes a curvature (often referred to as geometry) of the space-time of the universe.

The curvature of space depends on the value of Ω, the density parameter.

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Closed Universe

If Ω > 1 (i.e., the density is above the critical density), the geometry of space is closed like the surface of a sphere.

In a closed universe, gravity eventually stops the expansion of the universe, after which it starts to contract until all matter in the universe collapses to a point, a final singularity termed the "Big Crunch" – maybe!

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Open Universe

If Ω < 1 (i.e., the density is below the critical density), the geometry of space is open – negatively curved like the surface of a saddle.

An open universe expands forever, with gravity barely slowing the rate of expansion. The ultimate fate of an open universe is universal heat death, the "Big Freeze".

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Flat Universe

If Ω = 1 (i.e., the density is equal to the critical density), the geometry of space is flat – like a plane surface.

A flat universe expands forever but at a continually decelerating rate. The ultimate fate of the universe is the same as an open universe – a “Big Freeze”.(Note that we are talking about space-time, so the shapes at left are merely analogies in lower dimensions.)

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A Primeval "Cosmic Egg"?

In 1927, Georges Lemaître published a model of the universe suggesting that the universe might have originated when a primeval "cosmic egg" exploded in spectacular fireworks, creating an expanding universe.

Published in an obscure journal, it wasn't taken seriously at the time. But now, his contribution is highly valued.

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Discovery of Galactic Redshifts

In 1912, Vesto Slipher was the first to observe the shift of spectral lines of galaxies, making him the discoverer of galactic redshifts.

Redshifts are analogous to the Doppler effect – think racing cars or trains passing you at speed.

An observed redshift due to the Doppler effect occurs whenever a light source moves away from an observer.

Conversely, light sources moving towards an observer are blueshifted.

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More on Redshifts

You will often see a “z value” quoted as a measure of a redshift.

λobsv is the observed wavelength of a spectral line

λemit is the emission wavelength of that line

If z > 0, there is a redshift

If z < 0, there is a blueshift

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Hubble's Discovery

In 1928, Edwin Hubble found that the further the distance to a nebula, the greater the receding velocity of that nebula.

He used Cepheid variable stars as “standard candles” to estimate their distance, and measured their redshifts to estimate their velocity.

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Galactic Redshifts

Here are some examples of how spectral lines are shifted in stars and galaxies.

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Einstein's “Biggest Blunder”?

Evidence mounted that the universe was not static, but expanding.

This was consistent with the original Einstein model; Einstein could have predicted it, but had assumed the static universe was a given.

Einstein later remarked that the introduction of the cosmological constant was the biggest blunder of his life.

But was it? Wait a little while...

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“Big Bang” or Steady State?

There were two primary explanations put forth for the expansion of the universe:

» Lemaître's “Big Bang” theory, advocated and developed by George Gamow.

» A Steady State model, proposed in 1948 by Hermann Bondi, Thomas Gold, and Fred Hoyle, in which new matter would be created as the galaxies moved away from each other. In this model, the universe is roughly the same at any point in time.

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Zwicky's Discovery

In 1933, Bulgarian-born Swiss physicist Fritz Zwicky, while investigating the Coma cluster of galaxies, stumbled upon a major discrepancy between theory and observation.

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“Missing Mass?”

By studying the rotation of a galaxies within the Coma Cluster, Zwicky estimated that the visible mass of those galaxies was 400 times less than the mass needed to explain their rotational motion.

But Zwicky, while ahead of his time, was a pugnacious character, disliked by many of his colleagues, so his ideas were often not taken seriously.

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Vera Rubin's Discovery

In the late 1960s and early 1970s, Vera Rubin measured the velocities at which galaxies rotate, using a telescope at the Kitt Peak Observatory in Arizona,

She used a sensitive spectrometer to determine the spectrum of light coming from the stars in different parts of spiral galaxies.

She discovered something unexpected:

The stars far from the centers of galaxies, in the sparsely populated outer regions, were moving just as fast as those closer to the galaxy's center.

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Galactic Rotation

ViewView

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Zwicky was Right!

This was odd, because the visible mass of a galaxy does not have enough gravity to hold such rapidly moving stars in orbit.

It followed that there had to be a tremendous amount of unseen matter in the outer regions of galaxies where the visible stars are relatively few.

Rubin and her colleague Kent Ford went on to study some sixty spiral galaxies and always found the same thing.

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Explanation: “Dark Matter”

Rubin's observations and calculations showed that most galaxies must contain about ten times as much “dark” mass as can be accounted for by the visible stars.

Eventually other astronomers began to corroborate her work and it soon became well-established that most galaxies were in fact dominated by "dark matter":

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Why is it called “Dark” Matter?

Dark matter cannot be seen directly with telescopes; evidently it neither emits nor absorbs light or other electromagnetic radiation at any significant level.

Hence “dark” (as opposed to luminous) matter.

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Evidence for Dark Matter

A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a cluster of galaxies) between the source object and the observer.

Studies of many cases of lensing by galaxy clusters show evidence for large amounts of dark matter.

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Gravitational Lensing

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The Bullet Cluster

The Bullet Cluster: Hubble Space Telescope image with overlays.

The total projected mass distribution reconstructed from strong and weak gravitational lensing is shown in blue, while the X-ray emitting hot gas observed with the Chandra X-ray Observatory is shown in red.

The most direct observational evidence to date for dark matter is in a system known as the Bullet Cluster, a collision between two galaxy clusters.» X-ray observations show that much of the

baryonic matter (in the form of gas, or plasma) in the system is concentrated in the center of the system.

» However, weak gravitational lensing observations of the same system show that much of the mass resides outside of the central region of baryonic gas.

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Summary of Evidence

Observations of the rotational speed of spiral galaxies

The confinement of hot gas in galaxies and clusters of galaxies

The random motions of galaxies in clusters

The gravitational lensing of background objects, and

The observed fluctuations in the cosmic microwave background radiation

All require the presence of additional gravity, which can be explained by the existence of dark matter.

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Dark Matter Candidates

Dark matter candidates are usually categorized as:

Baryonic

Composed of baryons, i.e. protons and neutrons and combinations thereof.

Non-Baryonic

Hot Dark Matter (HDM)

Particles that have zero or near-zero mass, and so move relativistically.

Cold Dark Matter (CDM)

Particles sufficiently massive that they move at sub-relativistic velocities

Cosmological simulations with Cold Dark Matter and Warm Dark Matter. Halos selected at environments which could represent the Milky Way, the Andromeda nebula M31 and M33.

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MACHOs?

One potential baryonic form of dark matter is MACHOs (MAssive Compact Halo Objects):

A MACHO is a small chunk of normal baryonic matter, far smaller than a star, which drifts through interstellar space unassociated with any solar system.

Recent work has suggested that MACHOs are not likely to account for the large amounts of dark matter now known to be present in the universe

RAMBOs (Robust Association of Massive Baryonic Objects) have also been postulated.These are dark clusters of brown dwarfs or white dwarfs.

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Brown Dwarfs?

Stars with below 8% of the Sun's mass are called brown dwarfs. They are not hot enough to ignite the nuclear burning that keeps ordinary stars shining.

Other candidates for dark matter include:

Cold "planets" moving through interstellar space, unattached to any star, could exist in vast numbers without being detected

So could comet-like lumps of frozen hydrogen

So could black holes.

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WIMPs?

One potential non-baryonic form of dark matter is WIMPs (Weakly Interacting Massive Particles)

The main theoretical characteristics of a WIMP are:

Interaction only through the weak nuclear force and gravity

Large mass compared to standard particles

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Axions?

There are strong reasons for suspecting that dark matter isn't made of ordinary atoms at all. This argument is based on an isotope of hydrogen, deuterium (1 proton + 1 neutron). It turns out that if dark matter were made from ordinary atoms, then theory predicts that there should be much less deuterium in the Universe than we actually observe.

So, dark matter could consist of some form of 'exotic' particle.

One possibility is the Axion, a hypothetical particle whose existence would explain what is otherwise a puzzling feature of quantum chromodynamics (QCD), the leading theory of strong interactions.

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Neutrinos?

Another particle has been regarded as a candidate for dark matter: the elusive neutrino.

Neutrinos have no electric charge, and hardly interact at all with ordinary atoms: almost all neutrinos that hit the Earth go straight through it.

Because neutrinos so greatly outnumber atoms, they could make up the dominant dark matter, even if each weighed only a hundred millionth as much as an atom.

Experiments imply a non-zero mass for the neutrino, but one that is too small to account for much of the dark matter.

The best evidence for neutrino masses comes from the Super- Kamiokande experiment in Japan, which used a huge tank in a former zinc mine.

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SuperPartner Particles?

If Supersymmetry is realized in nature, every fermion in the SM must have a bosonic partner particle and vice versa.

No such “superpartner particle” has been observed so far, and recent LHC experiments have cast doubt on the theory.

Supersymmetry arises naturally from the combination of the two cornerstones of 20th century physics: quantum mechanics and relativity. It is the unique symmetry that relates the two fundamental kinds of particles:

Bosons, which act as the carriers of forces

Fermions, which act as the constituents of matter

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MOND?

In 1983, Mordehai Milgrom, a physicist (another Bulgarian-born!) at the Weizmann Institute in Israel, proposed Modified Newtonian dynamics (MOND), a modification of Newton's law of gravity, to explain the galaxy rotation problem.

While MOND provides an explanation for the observed galactic rotations, and has been extensively examined by many others, it does not appear to be consistent with other observations.

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So What is Dark Matter?

So, dark matter could be composed of any number of particles, both known and exotic:MACHOsWIMPsMassless neutrinosAxionsNeutralinosPhotinos

Or who knows what else?

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How Fast is the Universe Decelerating?

We have known since Hubble that the universe is expanding.

We know that gravity should cause this expansion to slow down, depending on how much matter is present in the universe.

If we measure this deceleration, we could determine the fate of the universe.

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Type Ia Supernovas

To do this, we need to find a set of 'standard candles' which can be used to determine the distance to extremely remote objects.

It turns out that one class of supernovae, Type Ia supernovae, can be used as standard candles.

A supernova results from the violent explosion of a white dwarf star.

Multi-wavelength X-ray / infrared image of SN 1572 or Tycho's Nova, the remnant of a Type Ia supernova

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Type Ia Supernova Creation

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Two Supernova Teams

In 1998/9, published observations of Type Ia supernovae by The High-z Supernova Search TeamThe Supernova Cosmology Project

suggested that the expansion of the universe is actually accelerating – a total surprise to everyone.

The 2011 Nobel Prize in Physics was awarded for this work.

Brian Schmidt, Saul Perlmutter, & Adam Riess.

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A New Paradigm of the Universe

So, it seems from all the evidence that the universe's evolution doesn't fit the original models!

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Corroboration of Results

Since then, these observations have been corroborated by several independent sources:Cosmic microwave background radiationGravitational lensing Large scale structure of the cosmosImproved measurements of supernovae

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Dark Energy

Evidence for Dark Matter and Dark Energy has accumulated, and it is now estimated that only about 4% of the matter/energy in the universe is 'ordinary matter'.

In other words, we have no real clue what the other 96% consists of!

This is most embarrassing!

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So, what is Dark Energy?

Candidates for Dark Energy include:Einstein's cosmological constant – dark energy is a property of space itself.An unidentified energy field, called “quintessence” – fills space like a fog and is similar to what drove inflationNone of the above – perhaps it's an illusion created by incorrect theories.

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Cosmological Constant?

Remember that Einstein thought it was his biggest blunder?

The Cosmological Constant has returned, and is the leading candidate for a Dark Energy explanation.

Maybe Einstein didn't blunder?

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A Slight Problem...

The Cosmological Constant being nonzero means that the vacuum can contain energy!

However, when physicists calculate the vacuum energy using our best theory, the Standard Model, they come up with an estimate that is 120 orders of magnitude (10120) too large!

This is even more embarrassing!

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Quintessence

The name Quintessence (“fifth essence”) dates back to the Ancient Greeks (Earth, Water, Fire, Air and...)

It has been proposed by some to be a fifth fundamental force.

The main difference between quintessence and the cosmological constant is that quintessence can vary with space and time.

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Implications

Cosmologists estimate that the acceleration began roughly 5 billion years ago.

Before that, it is thought that the expansion was decelerating, due to the attractive influence of dark matter and baryons.

The density of dark matter in an expanding universe decreases more quickly than dark energy, and eventually the dark energy dominates.

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The “Big Rip”

The Big Rip is a cosmological hypothesis first published in 2003, about the ultimate fate of the universe, based on phantom energy, an extreme form of quintessence.

It predicts that the matter of the universe will progressively be torn apart by the expansion of the universe at a certain time in the future.

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“The End of the Universe is Nigh!”

Don't worry!

It won't happen for billions of years.

We all have more immediate worries!

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Summary

We have significant evidence for large quantities of something in the universe we call:

Dark Matter

and

Dark Energy

And we don't really know what either of them are!