What is the

(1)

What is the Universe Made Of?

The case for Dark Matter and Dark Energy, and for what they might be Cliff Burgess

(2)

What is the Universe Made Of?

From best fits to the ‘Concordance Cosmology’

Courtesy: Ned Wright’s Cosmology Page

(3)

What is the Universe Made Of?

From best fits to the ‘Concordance Cosmology’

(4)

1905 – A Big Year for Einstein

• Photo-electric Effect

• “On a Heuristic Point of View concerning the Production and Transformation of Light.” rcd Mar 18, pub Jun 9

• Brownian Motion

• “On the Movement of Small Particles Suspended in Stationary Liquids Required by the Molecular-Kinetic Theory of Heat.” rcd May 11, pub Jul 18

• Special Relativity

• “On the Electrodynamics of Moving Bodies.” rcd Jun 30, pub 26 Sep

• Size of Molecules

• “A New Determination of Molecular Dimensions.” rcd Aug 19, pub Feb 8

• Mass-Energy Equivalence

• “Does the Inertia of a Body Depend upon Its Energy Content?” rcd Sep 27, pub Nov 21

Albert Einstein

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1905 – A Big Year for Einstein

• Photo-electric Effect

• “On a Heuristic Point of View concerning the Production and Transformation of Light.” rcd Mar 18, pub Jun 9

• Brownian Motion

• “On the Movement of Small Particles Suspended in Stationary Liquids Required by the Molecular-Kinetic Theory of Heat.” rcd May 11, pub Jul 18

• Special Relativity

• “On the Electrodynamics of Moving Bodies.” rcd Jun 30, pub 26 Sep

• Size of Molecules*

• “A New Determination of Molecular Dimensions.” rcd Aug 19, pub Feb 8

• Mass-Energy Equivalence

• “Does the Inertia of a Body Depend upon Its Energy

Content?” rcd Sep 27, pub Nov 21 * PhD Thesis and most cited

Albert Einstein

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Outline

• Dark Cosmology

• The Hot Big Bang

• Dark Matter

• Evidence for Dark Matter

• Dark stuff or modified gravity?

• Dark Energy

• Why doesn’t the vacuum gravitate?

• Dark Energy as vacuum energy

Taipei June 2014

(7)

Outline

• Dark Cosmology

• The Hot Big Bang

• Dark Matter

• Evidence for Dark Matter

• Dark stuff or modified gravity?

• Dark Energy

• Why doesn’t the vacuum gravitate?

• Dark Energy as vacuum energy

Taipei June 2014

(8)

Outline

• Dark Cosmology

• The Hot Big Bang

• Dark Matter

• Evidence for Dark Matter

• Dark stuff or modified gravity?

• Dark Energy

• Why doesn’t the vacuum gravitate?

• Dark Energy as vacuum energy

Taipei June 2014

(9)

Part I

“It is a capital mistake to theorize before one has data.

Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.”

Sherlock Holmes in A Scandal in Bohemia

(10)

The Hot Big Bang

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Gravity, Matter & Geometry

• According to Einstein gravity is really the response of space and time to the presence of matter.

• The presence of energy curves space and time.

• The curvature of space changes how objects move.

• Knowing how matter is distributed over large scales

tells us the shape and evolution of the Universe.

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Courtesy: Sloan Digital Sky Survey WMAP

Matter Distribution

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Evidence for an Expanding Universe

• The sky is dark

• The Hubble Law

• The homogeneity and isotropy of the

universe

• The slower decay of more distant

supernovae

v  H

₀

d

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Hot Big Bang

• Assume Universe once a hot soup of elementary particles

• Seek relics of earlier hotter epochs

• Use: at high temperatures

particles get broken to their

constituents. As universe

cools, bound states form

(15)

Hot Big Bang

• Atoms form below 1000 degrees

• electrons and nuclei combine into neutral atoms.

• Nuclei form below 10

¹⁰

degrees.

• Protons and neutrons combine into nuclei.

(16)

Evidence for a Hot Big Bang

• Primordial element abundances

• The cosmic microwave background

• T

_CMB

vs distance

Burles, Nolette & Turner, 1999

(17)

Evidence for a Hot Big Bang

• Primordial element abundances

• The cosmic microwave background

• T

_CMB

vs distance

Courtesy: Ned Wright’s Cosmology Page Burles, Nolette & Turner, 1999

Total Mass Density of Atoms

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Evidence for a Hot Big Bang

• Primordial element abundances

• The cosmic microwave background

• T

_CMB

vs distance

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Evidence for a Hot Big Bang

• Primordial element abundances

• The cosmic microwave background

• T

_CMB

vs distance

(20)

Evidence for a Hot Big Bang

• Primordial element abundances

• The cosmic microwave background

• T

_CMB

vs distance

(21)

Evidence for a Hot Big Bang

• Primordial element abundances

• The cosmic microwave background

• T

_CMB

vs distance

(22)

Evidence for a Hot Big Bang

• Primordial element abundances

• The cosmic microwave background

• T

_CMB

vs distance

(23)

Evidence for a Hot Big Bang

• Primordial element abundances

• The cosmic microwave background

• T

_CMB

vs distance

WMAP collaboration

CMB Temperature vs Direction

(24)

Evidence for a Hot Big Bang

Planck collaboration

CMB Temperature vs Direction

• Primordial element abundances

• The cosmic microwave background

• T

_CMB

vs distance

(25)

DARK MATTER

Part II

“Circumstantial evidence is a very tricky thing,” answered Holmes thoughtfully.

“It may seem to point very straight to one thing, but if you shift your own point of view a little, you may find it pointing in an equally uncompromising manner to something entirely different.”

Sherlock Holmes in The Boscombe Valley Mystery

(26)

DARK MATTER

The evidence for it

Dark stuff or modified gravity?

“Circumstantial evidence is a very tricky thing,” answered Holmes thoughtfully.

“It may seem to point very straight to one thing, but if you shift your own point of view a little, you may find it pointing in an equally uncompromising manner to something entirely different.”

Sherlock Holmes in The Boscombe Valley Mystery

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Evidence for Dark Matter

• Mass in galaxies

• Mass in clusters of galaxies

• Temperature

fluctuations in the CMB

• Start of galaxy

formation

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Evidence for Dark Matter

• Mass in galaxies

• Mass in clusters of galaxies

• Temperature

fluctuations in the CMB

• Start of galaxy

formation

(29)

Evidence for Dark Matter

• Mass in galaxies

• Mass in clusters of galaxies

• Temperature

fluctuations in the CMB

• Start of galaxy formation

(30)

Evidence for Dark Matter

• Mass in galaxies

• Mass in clusters of galaxies

• Temperature

fluctuations in the CMB

• Start of galaxy formation

The Bullet Cluster: Separating Dark and Visible Matter

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Evidence for Dark Matter

• Mass in galaxies

• Mass in clusters of galaxies

• Temperature

fluctuations in the CMB

• Start of galaxy formation

Colliding galaxy clusters

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Evidence for Dark Matter

• Mass in galaxies

• Mass in clusters of galaxies

• Temperature

fluctuations in the CMB

• Start of galaxy formation

Hot intra-cluster gas

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Evidence for Dark Matter

• Mass in galaxies

• Mass in clusters of galaxies

• Temperature

fluctuations in the CMB

• Start of galaxy formation

Mass measured by lensing

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Evidence for Dark Matter

• Mass in galaxies

• Mass in clusters of galaxies

• Temperature

fluctuations in the CMB

• Start of galaxy formation

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Evidence for Dark Matter

• Mass in galaxies

• Mass in clusters of galaxies

• Temperature

fluctuations in the CMB

• Start of galaxy formation

Time available for structure formation

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Evidence for Dark Matter

• Mass in galaxies

• Mass in clusters of galaxies

• Temperature

fluctuations in the CMB

• Start of galaxy formation

Time available for structure formation log r

log a

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Evidence for Dark Matter

• Mass in galaxies

• Mass in clusters of galaxies

• Temperature

fluctuations in the CMB

• Start of galaxy formation

log a

(38)

Evidence for Dark Matter

• Mass in galaxies

• Mass in clusters of galaxies

• Temperature

fluctuations in the CMB

• Start of galaxy formation

log a

(39)

What is the Dark Matter?

• Ordinary atoms?

• Modifications to the Law of

Gravity?

• New kind of

particles?

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What is the Dark Matter?

• Ordinary atoms?

• Modifications to the Law of

Gravity?

• New kind of particles?

Burles, Nolette & Turner, 1999

(41)

What is the Dark Matter?

• Ordinary atoms?

• Modifications to the Law of

Gravity?

• New kind of particles?

The devil is in the details:

No proposals yet succeed for galaxies and clusters and the CMB.

Very difficult to modify gravity at long distances without having problems with fundamental principles.

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What is the Dark Matter?

• Ordinary atoms?

• Modifications to the Law of

Gravity?

• New kind of particles?

Weakly Interacting Particles arise in most theories of

microscopic physics.

Their residual cosmic

abundance is naturally the right size to agree with the observed amount of

Dark Matter.

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DARK ENERGY

Part III

“How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?”

Sherlock Holmes in The Sign of the Four

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DARK ENERGY

Evidence for Dark Energy

The Cosmological Constant Problem

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Evidence for Dark Energy

• Brightness of very distant supernovae

• Flatness of the

universe as a

whole

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Evidence for Dark Energy

• Brightness of very distant supernovae

• Flatness of the universe as a whole

Very distant objects should not precisely follow Hubble’s Law because gravitational attraction should decelerate the universal expansion.

This can be tested by looking for deviations from Hubble’s Law for very distant supernovae.

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Evidence for Dark Energy

• Brightness of very distant supernovae

• Flatness of the

universe as a

whole

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Evidence for Dark Energy

• Brightness of very distant supernovae

• Flatness of the universe as a whole

The universal expansion should be decelerating due to gravitational attraction

Expect this:

(49)

Evidence for Dark Energy

• Brightness of very distant supernovae

• Flatness of the universe as a whole

Tonrey et.al., 2003

(50)

Evidence for Dark Energy

• Brightness of very distant supernovae

• Flatness of the universe as a whole

Tonrey et.al., 2003

Amount of Dark Matter

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Evidence for Dark Energy

WMAP collaboration Small temperature variations, at the level of one part in 100,000, are visible in the CMB

• Brightness of very distant supernovae

• Flatness of the

universe as a

whole

(52)

Evidence for Dark Energy

WMAP collaboration

These are due to sound waves in the primordial gas which emitted this light.

• Brightness of very distant supernovae

• Flatness of the

universe as a

whole

(53)

Evidence for Dark Energy

• Brightness of very distant supernovae

• Flatness of the universe as a whole

The CMB allows the inference of the properties of the later universe through which these photons pass.

(54)

Evidence for Dark Energy

• Brightness of very distant supernovae

• Flatness of the universe as a whole

Measurements of CMB and Dark Matter

and universal expansion and acceleration are consistent

Amount of Dark Matter

(55)

Concordance Cosmology

Can also count ordinary atoms even if they cannot be seen!

Nucleosynthesis

Properties of the CMB

(56)

The cosmological term

• Einstein’s equations as initially written preclude the existence of a static Universe

Taipei June 2014

𝐺

_𝜇𝜈

+ 𝜆 𝑔

_𝜇𝜈

= 8𝜋𝐺 𝑇

_𝜇𝜈

𝐺

_𝜇𝜈

= 8𝜋𝐺 𝑇

_𝜇𝜈

• This conclusion can be avoided if they are modified to include a ‘cosmological term’ which acts as a repulsive counterforce to gravity’s attraction

• The requirement for the cosmological term was removed once the Universe was found to be expanding.

(57)

Cosmological term as Dark Energy

• The cosmological term provides an excellent description of the Dark Energy, since its repulsive nature can drive the observed cosmological acceleration

• Interpreted as a stress-energy the cosmological term looks like constant positive energy density and negative pressure

8𝜋𝐺 𝑇

_𝜇𝜈

= −𝜆 𝑔

_𝜇𝜈

−𝑝 = 𝜌 = 𝜆

8𝜋𝐺

(58)

Einstein’s error

• Was Einstein’s greatest error introducing the cosmological term, or discarding it before Dark Energy was discovered?

Taipei June 2014

𝐺

_𝜇𝜈

= 8𝜋𝐺 𝑇

_𝜇𝜈

− 𝜆 𝑔

_𝜇𝜈

= 8𝜋𝐺 (𝑇

_𝜇𝜈

+ 𝑡

_𝜇𝜈

)

• Modern point of view: Neither! His error was to believe he gets to choose...

• The cosmological term is precisely what a vacuum energy, 𝑡_𝜇𝜈, would look like, and we should be able to compute its

properties if we understand the vacuum.

(59)

Vacuum Energy as Dark Energy

• The success of special relativity requires the

vacuum energy density to be constant and its pressure to be negative, as required to be Dark Energy.

• Negative pressure keeps the vacuum energy density

constant as the universe expands.

log r

log a

 0





 

r p

(60)

The Cosmological Constant Problem

• The vacuum energy is calculable within any theory of

elementary particles, such as the Standard Model of particle physics, and the observed vacuum energy is the sum of a classical energy and an enormous quantum energy

𝜌

_𝑣𝑎𝑐

= 𝜆 + 𝑚

⁴

4𝜋

²

• So what? Can always choose classical 𝜆 to ensure the

Universe accelerates by the right amount, even if 𝜌_𝑣𝑎𝑐 is much smaller than 𝑚⁴

(61)

Hierarchy problems

• The electroweak hierarchy

• The cosmological constant

𝐿

_𝑆𝑀

= 𝜇

²₀

+ 𝑚

²₀

𝐻

^∗

𝐻 + 𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛𝑙𝑒𝑠𝑠 𝜇

²

= 𝜇

²₀

+ ℎ𝑖𝑔ℎ𝑒𝑟 𝑜𝑟𝑑𝑒𝑟

m_e ~ 10⁶ eV

m_ 10^-2 eV m_w~10¹¹ eV

m_ ~ 10⁸ eV

Modern picture: no unique

‘classical’ theory; instead many ‘effective’ theories

𝜌

_𝑣𝑎𝑐

= 𝜆

₀

+ 𝑘

_𝜐

𝑚

_𝜐⁴

(62)

Hierarchy problems

• The electroweak hierarchy

• The cosmological constant

𝐿

_𝑆𝑀

= 𝜇

²₀

+ 𝑚

²₀

𝐻

^∗

𝐻 + 𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛𝑙𝑒𝑠𝑠 𝜇

²

= 𝜇

²₀

+ ℎ𝑖𝑔ℎ𝑒𝑟 𝑜𝑟𝑑𝑒𝑟

m_e ~ 10⁶ eV

m_ 10^-2 eV m_w~10¹¹ eV

m_ ~ 10⁸ eV

𝜌

_𝑣𝑎𝑐

= 𝜆

₁

+ 𝑘

_𝑒

𝑚

_𝑒⁴

+ 𝑘

_𝜈

𝑚

_𝜈⁴

𝜌

_𝑣𝑎𝑐

= 𝜆

₀

+ 𝑘

_𝜈

𝑚

_𝜈⁴

(63)

Hierarchy problems

• The electroweak hierarchy

• The cosmological constant

𝐿

_𝑆𝑀

= 𝜇

²₀

+ 𝑚

²₀

𝐻

^∗

𝐻 + 𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛𝑙𝑒𝑠𝑠 𝜇

²

= 𝜇

²₀

+ ℎ𝑖𝑔ℎ𝑒𝑟 𝑜𝑟𝑑𝑒𝑟

m_e ~ 10⁶ eV

m_ 10^-2 eV m_w~10¹¹ eV

m_ ~ 10⁸ eV

𝜌

_𝑣𝑎𝑐

= 𝜆

₁

+ 𝑘

_𝑒

𝑚

_𝑒⁴

+ 𝑘

_𝜈

𝑚

_𝜈⁴

𝜌

_𝑣𝑎𝑐

= 𝜆

₀

+ 𝑘

_𝜈

𝑚

_𝜈⁴

Must cancel to 32

decimal places!!

(64)

What We’re Looking For

• Our picture of the physics of ordinary particles must already be wrong at energies higher than 1 eV, or distances shorter than 1 micron.

• Whatever the change is, it must change gravity in such a way as to produce a small response to the vacuum energy.

• It must not alter other interactions.

• Is this possible? Party line says “no”.

(65)

What We’re Looking For

• Our picture of the physics of ordinary particles must already be wrong at energies higher than 1 eV, or distances shorter than 1 micron.

• Whatever the change is, it must change gravity in such a way as to produce a small response to the vacuum energy.

• It must not alter other interactions.

• Is this possible? Party line says “no”.

• Remarkably, it may be!

(66)

Helpful extra dimensions

• The Problem:

• Einstein’s equations make a lorentz-invariant vacuum energy (which is generically large) an obstruction to a close-to-flat spacetime (which we see around us)

𝑇

_𝜇𝜈

= 𝜆 𝑔

_𝜇𝜈

𝐺

_𝜇𝜈

= 8𝜋𝐺 𝑇

_𝜇𝜈

(67)

Helpful extra dimensions

• The Problem:

• Einstein’s equations make a lorentz-invariant vacuum energy (which is generically large) an obstruction to a close-to-flat spacetime (which we see around us)

𝑇

_𝜇𝜈

= 𝜆 𝑔

_𝜇𝜈

𝐺

_𝜇𝜈

= 8𝜋𝐺 𝑇

_𝜇𝜈

Arkani-Hamed et al Kachru et al Carroll & Guica Aghababaie et al

But this need not be true if there are

more than 4 dimensions!!

(68)

Helpful extra dimensions

• Why not?

• Extra dimensions need not be lorentz invariant

• Vacuum energy might curve extra dimensions, rather than the ones we see in cosmology

Vilenkin et al

e.g. gravitational field of a cosmic string

(69)

Helpful extra dimensions

• A higher-dimensional analog:

• Similar (classical) examples also with a 4D brane in two extra dimensions: e.g. the rugby ball and related solutions

Carroll & Guica Aghababaie et al

(70)

Opportunities & Concerns

• If true, many striking implications:

• Micron deviations from inverse square law

• Missing energy at the LHC and in astrophysics:

requires M_g > 10 TeV

• Probably a vanilla SM Higgs

• Excited string states (or QG) at LHC below 10 TeV

• Low energy SUSY without the MSSM

• Very light Brans-Dicke-like scalars

• Sterile neutrinos from the bulk?

(71)

“…when you have eliminated the impossible, whatever remains, however improbable, must be the truth.”

A. Conan Doyle

(72)

Outlook

• Cosmological observations are now redundantly testing the Hot Big Bang model.

• Observations support the ‘Concordance Cosmology’.

(73)

Outlook

• Cosmological observations are now redundantly testing the Hot Big Bang model.

• The concordance involves several lines of independent evidence for both Dark Matter and Dark Energy.

• Neither can be dark forms of ordinary atoms.

(74)

Outlook

• Cosmological observations are now redundantly testing the Hot Big Bang model.

• The concordance involves several lines of independent evidence for both Dark Matter and Dark Energy.

• Neither can be dark forms of ordinary atoms.

• Dark Matter likely new particle types, but Dark Energy is harder (until recently thought impossible).

• If so we’ll know from a variety of observational tests.