Dark Energyand its Discontents

BLOG: http://telescoper.wordpress.com

Peter ColesUniversity of Sussex

Big Bang Basics

The Hubble Expansion

THE COSMIC WEB

The Stretching of Light!

2dFGRS

UDS

The Cosmic Microwave Background

• Discovered by accident by Penzias & Wilson (1965); Nobel Prize in 1978.

• Thermal (“black body”) radiation with a temperature about 3 degrees above absolute zero

• This radiation was produced when the Universe was 1000 times smaller, and 1000 times hotter

• The “Smoking Gun” of the Big Bang

The State of the Universe

The Big Bang Theory is not a Theory: it’s a Model..

• There are infinitely many possible big bang universes described by the same equations, but with different initial conditions.

• Some expand forever, some recollapse.• We have to use observations to pick

which of the big bang family is closest to our universe.

Theories

Observations

FrequentistBayesian

April 28, 2015

Planck Time!

April 28, 2015

BICEP2

“CONCORDANCE”

Ingredients of the Standard Cosmology

•General Relativity •Cold Dark Matter

•Cosmological Constant•Cosmological Principle

•Primordial Gaussian fluctuations•Inflation•Baryons•Neutrinos•Radiation…

Questionable Aspects of the Standard Cosmology

•General Relativity •Cold Dark Matter

•Cosmological Constant•Cosmological Principle

•Primordial Gaussian fluctuations•Inflation•Baryons•Neutrinos•Radiation…

SAY “PRECISION COSMOLOGY”

ONE MORE TIME...

Precision Cosmology

“…as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns -- the ones we don't know we don't know.”

Three Pillars of Wisdom

Dark Matter…..

• Most of the matter in the Universe is dark..

• “Dark Matter” can be detected in galaxies and galaxy clusters.

• Observations suggest Ω0≈0.25, not enough to close the Universe

• But the Universe appears flat...• ….and it is accelerating!

fainter

Nobel Prize for Physics 2011

Reasons for Dark Energy

• Galaxies suggest dark matter is only 20 to 30% of what is needed to make space flat

• The Cosmic Microwave Background tells us space is flat

• Supernovae tell is that the Universe can’t be flat and dominated by dark matter

• .

The Accelerating Universe

• Gravity pulls “normal” matter and energy; it doesn’t push.

• It is possible to engineer anti-gravity, but only with peculiar energy called “Vacuum Energy” or “Dark Energy”

• This is the idea behind inflation in the early Universe ..

• ..and possibly why the Universe accelerates now

Dark Energy

• The “Cosmological Constant” (Λ) was introduced by Einstein to make a static universe (his “biggest blunder”)

• Now we think of Λ as “vacuum energy” that causes the expansion Universe to speed up..

• The vacuum energy is directly connected to the microscopic physics of elementary particles.

Dark Energy:A Cosmologist’s

Perspective

TG =

Gravity Stuff

TG =+?

Dark Energy:A Physicist’s Perspective

?+=TG

Whatever it is, Dark Energy is a terrible name

for it…

•What is important is not so much the energy, but the pressure…•Dark Energy has to act like something with negative pressure (or tension)•It also has to be smoothly distributed throughout space

Alternatives

•Modified Gravity•Back-reaction of structure•Is Dark Energy Dynamical?•None of the above….

Conclusions

• The standard cosmology is a great success, but we don’t know what most of the Universe is made of.

• Maybe we are thinking about things the wrong way entirely

• The biggest problem now is theoretical, not observational