dark energy and cosmic sound

Daniel Eisenstein – Univ. of Arizona

Dark Energy and Cosmic Sound

• Bob Nichol on behalf of the SDSS Collaboration

• Copy of presentation to be given by Daniel Eisenstein at AAS meeting in San Diego at 9am PST today

Bob Nichol - ICG, Portsmouth


Synopsis

•evidence that gravity played the major role in the growth of galaxies: the “Smoking gun”“Smoking gun”

•we have detected the “acoustic oscillations”, a sound wave created in the first minute of the Universe

•use “oscillation” as a “cosmic yardstick” to measure distances more accurately than before.

•the most precise measurement yet of the geometry of the Universe: it’s flat to 1%!



How did galaxies form?

• Local universe is full of galaxies: Where did they come from? How did they form they way they did?

• Our theory predicts they form via gravity from tiny quantum fluctuations in the early Universe



Sound Waves in Early Universe

• For 400,000 years after the Big Bang, the cosmic microwave background photons are trapped in the ionized cosmic gas.

• These photons provide an enormous restoring pressure, causing the gas to resist being squeezed by gravity

• Therefore, fluctuations in the gas propagate as sound wavessound waves.

• This ends abruptly when the Universe cools below 3000K and the gas becomes neutral: Universe suddenly becomes transparent

Big

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Recombinationz ~ 1000

~400,000 yearsIonized Neutral

Time



Cosmic Microwave Background

• Effect of this sound wave already discovered in relic light of the early universe i.e. the CMB!

• That was the Universe at 400,000 years. Can we see these sound waves today?



Theory of the sound wave• At first, sound wave expands at 57% of the speed of light, then slows as the gas changes from ionized to neutral (red = ionized, green = neutral)

• Final size is reached after one million years. Today, that radius is 500 million 500 million light yearslight years.

• Central peak is overdense in dark matter. Outer ring is overdense in gas. Both are Both are seeds for the formation of seeds for the formation of galaxiesgalaxies.Our theory accurately Our theory accurately predictspredicts an excess of galaxy pairs an excess of galaxy pairs

separated by 500 million light-years: this would be the separated by 500 million light-years: this would be the “SMOKING GUN” “SMOKING GUN” that only gravity was important to explain that only gravity was important to explain the rich structures of galaxies and clusters of galaxies we the rich structures of galaxies and clusters of galaxies we see todaysee today Bob Nichol - ICG,

Portsmouth

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The Sloan Digital Sky Survey• The Sloan Digital Sky Survey

is a survey of (one quarter) of the northern sky. Over 200 scientists from 14 institutions around the world.

• Here we report on a sample that covers 10% of the sky.

• Luminous red galaxy sample: special spectroscopic sample of 47,000 galaxies that extends to z = 0.47, 6 billion light years away.

SDSS Telescope in Apache Point, New Mexico


Largest volume ever surveyed Largest volume ever surveyed with galaxies (This is why we with galaxies (This is why we

can see this wave)can see this wave)


700,000 light years

500 Million Light Years

Credit: SDSSA slice of the SDSS



The Correlation Function

• The correlation function is the probability of finding pairs at a given separation, above that of a random distribution.

Excess of galaxies separated by 500 million

light years



What does it mean?

• We have detected the sound wave in the Universe at two very different epochs (400,000 yrs after Big Bang and present-day). This is important because our theory of gravitational structure formation predicts that such features should have been preserved. Detecting the sound wave in the galaxies is the “SMOKING GUN”SMOKING GUN” that our theory is correct.

• Better yet, the sound wave is an object of fixed size, a “standard ruler” or “cosmic yardstick”. This means that we can measure its apparent size anywhere in the Universe, and determine how far it is away because we know its true size.



Looking back in time in the Universe

FLAT GEOMETRYCREDIT: WMAP & SDSS websites

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FLAT GEOMETRY

SD

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LAX

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SD

SS

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LAX

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CM

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OPEN GEOMETRY




CM

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CLOSED GEOMETRY

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UNIVERSE IS FLAT TO 1% UNIVERSE IS FLAT TO 1% PRECISIONPRECISION



Dark Energy

• In 1998, two groups used distant supernovae to discover the acceleration of the expansion history of the Universe.

• supernovae were fainter than expected, implying that they were further away.

• The cause of this is completely unknown but almost surely exotic new physics. It has been dubbed “dark dark energyenergy”. Our detection requires dark energy to be correct!

• Now we can map the expansion of the Universe using our “cosmic yardstick”. This is a robust and innovative new method for cosmologists and will spawn future surveys of the Universe.



More Information

• http://cmb.as.arizona.edu/~eisenste/acousticpeak

• http://www.dsg.port.ac.uk/~nicholb/wiggles

• [email protected]



Thanks to....

• The science analysis team was supported by several grants from the National Science Foundation, as well as funds from the University of Arizona, the Sloan Foundation, and NASA.

• The SDSS is supported by the Alfred P. Sloan Foundation, NASA, the National Science Foundation, Dept of Energy, the Japanese Monbukagakusho, the Max Planck Society, and the Participating Institutions: University of Chicago, Fermilab, the Institute for Advanced Study, the Japanese Participation Group, the Johns Hopkins University, the Korean Scientist Group, the Max Planck Institute for Astronomy, the Max Planck Institute for Astrophysics, New Mexico State University, the University of Pittsburgh, the University of Portsmouth, Princeton University, the US Naval Observatory, and the University of Washington.



EXTRA SLIDES

• EXTRA SLIDES FOR Q&A



What is going on (part 2) ?

•Gravity squeezes the gas, pressure pushes back! They oscillate

•When the Universe cools below 3000K these sound waves are frozen in


Courtesy of Wayne Hu


Cosmic Yardstick?


If we know the true size of something, we can estimate how far it is away by it’s apparent size

Satellite photos of Earth via

Google can now be used to measure distances


Color SDSS Data (Hogg & Blanton)

LRG


Life is more complicated•Universe is composed of many perturbations, all superimposed.

•We do not expect to see bulls-eyes in the galaxy map. Indeed, the ring is only 1% of the height of the center.

•The whole analysis is statistical. Therefore need massive datasets like SDSS


dark energy and cosmic sound

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