Cosmic Acceleration

1) The evidence

2) What it could be

3) Why each options is interesting/confusing

4) Planning the next steps

1) The evidence

- Supernovae

- CMB

- Clusters

1) The evidence

- Supernovae

- CMB

- Clusters

The Hubble law

v Hr3 / sec

100

mH

lightyears

v

r

Cosmic acceleration

Using supernovae (exploding stars) as cosmic “mileposts”, acceleration of the Universe has been detected.

Supernova

Supernova video (links provided in online course notes)

Acceleration of the universe

The Hubble law at great distances depends on the variations of the Hubble “constant” H with time.

" "v

" "r

Supernova

Preferred by data c. 2003

Amount of gravitating matter A

mount

of

acc

lera

ting m

att

er

(“D

ark

energ

y”)

“Ordinary” non accelerating matter

Cosmic acceleration

Accelerating matter is required to fit current data

Cosmic acceleration

Accelerating matter is required to fit current data

Supernova

“Ordinary” non accelerating matter

Preferred by data c. 2008

BAO

Kowalski, et al., Ap.J.. (2008)

Amount of gravitating matter A

mount

of

acc

lera

ting m

att

er

(“D

ark

energ

y”)

Cosmic acceleration

Accelerating matter is required to fit current data

Supernova

“Ordinary” non accelerating matter

Preferred by data c. 2008

BAO

Kowalski, et al., Ap.J.. (2008)

(Includes dark matter)

Amount of gravitating matter A

mount

of

acc

lera

ting m

att

er

(“D

ark

energ

y”)

1) The evidence

- Supernovae

- CMB

- Clusters

1) The evidence

- Supernovae

- CMB

- Clusters

The Edge of the Observable Universe:

As we look back in space we look back in time. We see:

Here & Now

Light traveling from far away =from distant past

Long ago (about 14 Billion years) the Universe was so hot and dense it was opaque: The edge of the observable universe

Today:• Only 2.726K above absolute Zero

• “Microwave Radiation” (The “Cosmic Microwave Background”: CMB)

• 1,000,000 times weaker than ambient radiation in a pitch dark room.

Properties of the Edge of the Observable Universe:

Here & Now

Similar to surface of Sun at time of emission

(~ 6000 )K

Observing the Microwave Background, Past, present and future:

The History of the Universe Time

High Energy & Temp

New Image of the “Last Scattering Surface” from NASA’s WMAP satellite released Feb 11 2003

WMAP map of the “edge of the

observable universe”

plotted as a sphere

Note: we are really on the inside looking

out

• Characteristic oscillations in the CMB power

Adapted from

Bennett et al Feb 11 ‘03

WMAP

“Active” models

Inflation

I.1 Successes

Tem

pera

ture

Pow

er

Angular scale

http://space.mit.edu/home/tegmark/movies.html

The predicted “cmb power” curves are different for different models of the universe. Only curves with specific parameter choices fit the data.

Cosmic acceleration Using supernovae (exploding stars) as cosmic “mileposts”,

acceleration of the Universe has been detected.

Supernova

Preferred by modern data

Amount of gravitating matter

A

mount

of

“anti

gra

vit

y”

matt

er

“Gravitating” non accelerating matter

1) The evidence

- Supernovae

- CMB

- Clusters

1) The evidence

- Supernovae

- CMB

- Clusters

Dark matter require to explain galaxy rotation curve data

“Cluster data” measures something similar

Cosmic acceleration (newest data)

Using supernovae (exploding stars) as cosmic “mileposts”, acceleration of the Universe has been detected.

Supernova

Preferred by modern data

Amount of gravitating matter

A

mount

of

“anti

gra

vit

y”

matt

er

“Gravitating” non accelerating matter

Supernova

Preferred by data c. 2003

Amount of “ordinary” gravitating matter A

mount

of

w=

-1 m

att

er

(“D

ark

energ

y”)

“Ordinary” non accelerating matter

Cosmic acceleration

Accelerating matter is required to fit current data

Cosmic acceleration

Accelerating matter is required to fit current data

Supernova Amount of “ordinary” gravitating matter A

mount

of

w=

-1 m

att

er

(“D

ark

energ

y”)

“Ordinary” non accelerating matter

Preferred by data c. 2008

BAO

Kowalski, et al., Ap.J.. (2008)

Cosmic acceleration

Accelerating matter is required to fit current data

Supernova Amount of “ordinary” gravitating matter A

mount

of

w=

-1 m

att

er

(“D

ark

energ

y”)

“Ordinary” non accelerating matter

Preferred by data c. 2008

BAO

Kowalski, et al., Ap.J.. (2008)

(Includes dark matter)

Supernova

Preferred by modern data

Amount of gravitating matter

A

mount

of

“anti

gra

vit

y”

matt

er

“Gravitating” non accelerating matter

Here for inflation

Accelerating “Dark Energy” is what makes U=1 (required to give consistency with inflation)

Acceleration or (required for inflation) is possible (+)

Dark Energy *very* poorly understood (-/+)

Dark Energy and Inflation

Supernova

Preferred by modern data

Amount of gravitating matter

A

mount

of

“anti

gra

vit

y”

matt

er

“Gravitating” non accelerating matter

Dark Energy and the fate of the Universe

In the presence of dark energy, the simple connection between open/closed/flat and the future of the universe no longer holds

Dark Energy (accelerating)

70%

Dark Matter 25%

Ordinary Matter (observed in labs)

5%

95% of the cosmic matter/energy is a mystery. It has never been observed even in our best laboratories

Dark Energy (accelerating)

70%

Dark Matter 25%

Ordinary Matter (observed in labs)

5%

95% of the cosmic matter/energy is a mystery. It has never been observed even in our best laboratories

Gravitating

Problems with cosmic acceleration

-Shouldn’t the pull of gravity slow down the expansion?

-The challenge is much greater when one knows the “foundations” of particle physics. Extremely difficult to accommodate acceleration

Cosmic Acceleration

1) The evidence

2) What it could be

3) Why each options is interesting/confusing

4) Planning the next steps

What could cause the acceleration?

1) A “cosmological constant” (see “special topic” on p698)

2) A new batch of “potential dominated matter”. Can think of the current era as “starting a new period of inflation”.

3) We must develop a new theory of gravity which can account for the data without needing acceleration (in the above, we assume Einstein’s General Relativity is correct).

4) Some other misinterpretation of data? (Getting less and less likely as more data comes in)

What could cause the acceleration?

1) A “cosmological constant” (see “special topic” on p698)

2) A new batch of “potential dominated matter”. Can think of the current era as “starting a new period of inflation”.

3) We must develop a new theory of gravity which can account for the data without needing acceleration (in the above, we assume Einstein’s General Relativity is correct).

4) Some other misinterpretation of data? (Getting less and less likely as more data comes in)

What could cause the acceleration?

1) A “cosmological constant” (see “special topic” on p698)

2) A new batch of “potential dominated matter”. Can think of the current era as “starting a new period of inflation”.

3) We must develop a new theory of gravity which can account for the data without needing acceleration (in the above, we assume Einstein’s General Relativity is correct).

4) Some other misinterpretation of data? (Getting less and less likely as more data comes in)

What could cause the acceleration?

1) A “cosmological constant” (see “special topic” on p698)

2) A new batch of “potential dominated matter”. Can think of the current era as “starting a new period of inflation”.

3) We must develop a new theory of gravity which can account for the data without needing acceleration (in the above, we assume Einstein’s General Relativity is correct).

4) Some other misinterpretation of data? (Getting less and less likely as more data comes in)

Each of the above explanations has its problems:

Cosmic Acceleration

1) The evidence

2) What it could be

3) Why each options is interesting/confusing

4) Planning the next steps

Each of the above explanations has its problems:

1) A “cosmological constant”: Requires a value 10-120 the “natural value”. Hard to know how that could come about.

Each of the above explanations has its problems:

1) A “cosmological constant”: Requires a value 10-120 the “natural value”. Hard to know how that could come about.

-A “horizon” forms around us as the acceleration continues.

-We will never see any object cross the horizon (just a bit larger than the currently observed universe)

- Perhaps the universe is truly finite?

Each of the above explanations has its problems:

1) A “cosmological constant”: Requires a value 10-120 the “natural value”. Hard to know how that could come about.

-A “horizon” forms around us as the acceleration continues.

-We will never see any object cross the horizon (just a bit larger than the currently observed universe)

- Perhaps the universe is truly finite?

Much like a black hole horizon

Each of the above explanations has its problems:

1) A “cosmological constant”: Requires a value 10-120 the “natural value”. Hard to know how that could come about.

-A “horizon” forms around us as the acceleration continues.

-We will never see any object cross the horizon (just a bit larger than the currently observed universe)

- Perhaps the universe is truly finite?

Much like a black hole horizon

Long list of amazing implications!

Each of the above explanations has its problems:

2) A new batch of “potential dominated matter”. Requires a new elementary particle with mass 10-33 times the electron mass. Tough to fit that into current theories.

Each of the above explanations has its problems:

3) We must develop a new theory of gravity which can account for the data without needing acceleration (in the above, we assume Einstein’s General Relativity is correct).

Pretty much every attempt to do this has produced a problematic theory (i.e. mathematically inconsistent).

Each of the above explanations has its problems:

4) Some other misinterpretation of data? So far the most compelling suggestions have all been ruled out.

American Association for the Advancement of Science

American Association for the Advancement of Science

…at the moment, the nature of dark energy is arguably the murkiest question in physics--and the one that, when answered, may shed the most light.

“Right now, not only for cosmology but for elementary particle theory, this is the bone in our throat.” - Steven Weinberg

“… Maybe the most fundamentally mysterious thing in basic science.” - Frank Wilczek

“… would be No. 1 on my list of things to figure out.” - Edward Witten

“Basically, people don’t have a clue as to how to solve this problem.” - Jeff Harvey

‘This is the biggest embarrassment in theoretical physics” - Michael Turner

From P 680 of text (same as 5e)

From P 677 of text (compare with 5e!)

What is the source of our textbook’s skepticism?

That we don’t really know if we are interpreting the data correctly, so “dark energy” and “acceleration” may not really be the right description?

AA: Fair enough!

Are the hoping that new data will make the phenomenon “go away”, removing the need for some new piece of our theory of the cosmos?

AA: If so, they are behind the times!

University of Chicago10 May 2006

Andreas Albrecht

The report from the Dark Energy Task Force

ContextContextContextContext Dark energy appears to be the dominant component of the physical

Universe, yet there is no persuasive theoretical explanation. The

acceleration of the Universe is, along with dark matter, the observed

phenomenon which most directly demonstrates that our fundamental

theories of particles and gravity are either incorrect or incomplete. Most

experts believe that nothing short of a revolution in our understanding of

fundamental physics will be required to achieve a full understanding of the

cosmic acceleration. For these reasons, the nature of dark energy ranks

among the very most compelling of all outstanding problems in physical

science. These circumstances demand an ambitious observational

program to determine the dark energy properties as well as possible.

10

ContextContextContextContext Dark energy appears to be the dominant component of the physical

Universe, yet there is no persuasive theoretical explanation. The

acceleration of the Universe is, along with dark matter, the observed

phenomenon which most directly demonstrates that our fundamental

theories of particles and gravity are either incorrect or incomplete. Most

experts believe that nothing short of a revolution in our understanding of

fundamental physics will be required to achieve a full understanding of the

cosmic acceleration. For these reasons, the nature of dark energy ranks

among the very most compelling of all outstanding problems in physical

science. These circumstances demand an ambitious observational

program to determine the dark energy properties as well as possible.

10

#1 on Science magazine’s list of “most compelling puzzles and questions facing

scientists today”

Goals and MethodologyGoals and MethodologyGoals and MethodologyGoals and Methodology1. The goal of dark-energy science is to determine the very nature of the dark

energy that causes the Universe to accelerate and seems to comprisemost of the mass-energy of the Universe.

2. Toward this goal, our observational program must:a. Determine as well as possible whether the accelerated expansion is

consistent with being due to a cosmological constant.b. If it is not due to a constant, probe the underlying dynamics by

measuring as well as possible the time evolution of dark energy, for example by measuring w(a); our parameterization

w(a) w wa( a).

c. Search for a possible failure of GR through comparison of cosmicexpansion with growth of structure.

3. Goals of dark-energy observational program through measurement of expansion history of Universe [dL(z) , dA(z) , V(z)], and through measurement

of growth rate of structure. All described by w(a). If failure of GR, possibledifference in w(a) inferred from different types of data.

Supernova

Preferred by modern data

Amount of ordinary matter

A

mount

of

“anti

gra

vit

y”

matt

er

“Ordinary” non accelerating matter

Here for inflation

Proposed new experiment

The SNAP Satellite

Another instrument that can vastly improve our knowledge of dark energy

• The LSST (Large-aperture Synoptic Survey Telescope) NB: the director of LSST is Prof Tony Tyson of UCD