Dark Energy & overview of Cosmological parameters

Today:Review last timeRevisit the accelerating universeLook at values of cosmological parameters

from different methodsDiscuss the final exam and review lecture

Last Time

• We heard about methods used to determine the ultimate fate of the universe – average density of matter– expansion rate in the distant past– large-scale geometrical properties of universe– motions of galaxies/clusters relative to smooth

expansion– abundances of light elements synthesized

shortly after the Big Bang

Last Time

• Most successful recent technique has been determining expansion rate as a function of time by measuring the apparent brightness of distant supernovae

• We discussed this a bit back in lecture 8

Type 1A Supernovae

SNe 1a

• What produces a SNe1a?– Start off with a binary star system– One star comes to end of its life – forms a “white

dwarf” (made of helium, or carbon/oxygen)– White Dwarf starts to pull matter off other star… this

adds to mass of white dwarf (accretion)– White dwarfs have a maximum possible mass… the

Chandrasekar Mass (1.4 MSun)– If accretion pushes White Dwarf over the Chandrasekar

Mass, it starts to collapse.

An accreting White Dwarf

SNe 1a• White Dwarf starts to collapse…

– Rapidly compresses matter in white dwarf– Initiated runaway thermonuclear reactions – star turns

to iron/nickel in few seconds– Liberated energy blows star apart– Resulting explosion briefly outshines rest of galaxy

containing it… these are the SNe1a events

• SNe1a– No remnant (neutron star or black hole) left– Since white dwarf always has same mass when it

exploded, these are “standard candles” (i.e. bombs with a fixed yield)

Hubble’s constant with SNe1a’s

• The program:– Search for SNe1a in distant

galaxies– Compare expected power

with observed power to determine distance

– Measure velocity using redshift

• “Low redshift” galaxies give measurement of H0

• “High redshift” galaxies allows you to look for deceleration of universe

The results…

• This program gives Hubble’s constant– H=65 km/s/Mpc

• and find acceleration, not deceleration!– Very subtle, but really is

there in the data!

– Profound result!

What are we trying to do ?

Filippenko & Riess (2000)

Low-z SNe Ia

High-z SNe Ia ?

If expansion rate does not change w/ time, expect all the data to lie along here

If expansion rate was lower in the past - distant objects should be closer than the H=constant case So data will be here

Further/fainter than expected

Brighter /closerthan expected

So, where’s the data?

Filippenko & Riess (2000)

=1,=0,

q=0.5

We see Acceleration !

Further/fainter than expected

Sne Ia for Cosmology (z > 0.1)

In the mid/late 1990s, as SNe 1a were detected out to greater distances (z > 0.1)

became clear that the observed fluxes of SNe 1a at 0.3 < z < 0.8 were systematically lower than expected

eg. for a critical density (matter-dominated) universe, & even for a constant-velocity (empty of matter) universe. The SNe must have gotten farther than expected for some epoch of the universe The universe appears to have been accelerating between the epoch equivalent to z ~ 0.5 and now !

i.e. expanding faster and faster !

The cosmic concordance

• What is our universe like?– Geometry (flat, spherical, hyperbolic)?– Matter content?– Anything else strange?

• Remarkable agreement between very different experimental techniques

• Cosmologists, it seems (amazingly!), really do know what they’re talking about!

Measurements of the matter content of the Universe (recap)

• Primordial nucleosynthesis– Theory predicts elemental abundances as a function of

baryon density– Observed abundances B0.036

• Galaxy/galaxy-cluster dynamics– Look at motions of stars in galaxies, or galaxies in

galaxy-clusters…– Infer presence of large quantities of “dark” matter

which is gravitationally affected observed objects.

from DM

• Analysis of galaxy motions suggests a dark matter density of DM0.3

• Same conclusion from gravitational lensing by clusters

estimates

• First stunning conclusion:– Compare B0.036 and DM0.3

– Normal matter only accounts for about 1/8 of the matter that’s out there!

– We’re made of the minority stuff!

estimates• Confirmed by taking

an inventory of a cluster…– Find that about 1/8 of a

cluster’s mass is in baryons

– We believe that clusters should be representative samples of the universe…

– Confirms DM0.3

Geometry

• Distribution of galaxies versus z shows…– The universe is so close to flat we can’t see any

deviations

The accelerating Universe

• Back to those observations of Type-1a Supernovae (SN1a)– Very good “standard candles”– Can use them to measure relative distances very

accurately– they show the universe expansion is

accelerating

Dark Energy

• There is an “energy” in the Universe that is making it accelerate– Call this “Dark Energy”– This makes up the rest of the gravitating energy in the

Universe, and causes it to be flat!– Completely distinct from “Dark Matter”

• Remember Einstein’s cosmological constant…?– Dark Energy has precisely the same effect as Einstein’s

cosmological constant– So, he was probably right all along!

Inflationary Cosmology• Dark Energy, inflationary cosmology, universe all refer

to this accelerating expansion

• Recall the period of inflation in the early universe– that had the universe undergoing a brief period of exponential

expansion-perhaps due to vast energy liberated by a symmetry break

• Inflationary Cosmology has the same thing happening …galaxy separations increasing with time (not so dramatic as the initial period of inflation)

Inflation/Dark EnergySolves 3 problems with the Big

Bang Model:

Horizon ProblemWe worried about how long things were causally-connected and could equilibrate, need equilibrium to explain uniformity of CBR - include and things could equilibrate and then expand suddenly…so OK

Dark EnergySolves 3 problems with the Big Bang Model:

The Structure problem Models match observed structure better than simple BB

modelsThe Relic problem

Theories had predicted production of particles and other stuff which should be around today, but which are not observed, e.g. the magnetic monopole. Predictions from inflationary cosmology match observation better

The magnetic monopole density is v. small under inflation, so lack of detection expected

Dark Energy

• Read Chapter 15 for some more details….

INVENTORY OF THE UNIVERSE

• So… our best guess for the inventory of the Universe is:– Small fraction is radiation we are familiar with.

– 3.6% is baryonic matter

– About 30% is dark matter

– Rest (about 70%) is dark energy!

• Second stunning fact… most of the Universe isn’t even made of matter! BDM1

Energy/Matter Density

• Results… get good agreement between theory and observed structure if: DM0.3 (mostly “cold” dark matter)

0.7

• Remarkable agreement with SNe1a results (but a completely independent method!).

The Age of the Universe

• Using this cosmological model, we can figure out the age of the Universe.– Answer – 13 to 14 Gyr

• Prediction…– There should be no object in the Universe that is older than

13 or 14 Gyr.– This agrees with what’s seen… there’s no object in the

Universe that appears to be older than this!– This was a big problem with old cosmological models

that didn’t include dark energy.

Again, what is “dark energy”?

• An “energy” that is an inherent component of space…

• Consider a region of vacuum– Take away all of the radiation– Take away all of the matter– What’s left? Dark energy!– But we have little idea what it is…

Final Exam

Thursday May 15 10:30am - 12:30pm

Room 107 Closed Book Bring a calculator

Final ExamExcluded topics:Telescope sessionLuminosity functions of planetary nebulae & globular clustersFar distant fate of the universe (details out to 10100 years)

Included topics:Everything else, note, ancient cosmology (which was excluded from the mid-term) is in the final exam

You will have a section of straightforward questions to test breadth of knowledge plus section of more involved questions for depth of knowledge (some choice of questions)