HOT TOPICS IN A HOT BIG BANGJohn Webb, School of Physics, UNSW

Sutherland Astronomical SocietyThursday 9th October 2008

This Powerpoint presentation will be made available at

http://www.phys.unsw.edu.au/~jkw/Sutherland/

Cosmology is in great shape. Big Bang nucleosynthesis calculations give fabulous agreement with observations, entirely consistent with the (independent) Cosmic Microwave Background observations. Large telescopes fill in the details but the basic concepts are now established beyond any doubt. So it’s all sorted out.

OR IS IT?

And yet...the interpretation forced upon us by Einstein's equations and some simplifying assumptions is that 73% of the energy density Universe appears to be in the form of a "dark energy" energy whose (negative gravity) presence we can so far only observe indirectly but its influence on the dynamics of the Universe. Further, 23% of the energy density appears to be "dark matter", which again has only been detected via dynamical observations. You and I comprise the remaining, unimpressive, 4%.

IT’S ALL RATHER EMBASSARRING!

This talk addresses the above issues and also examines a new fundamental result I Cosmology: a possible detection of the “Epoch of Re-ionisation”.

Big bang nucleosynthesis and

the light element abundances

• H, 4He, 3He, 7Li and D (2H) were all produced during BBN.

• The amount of each element produced is dependent on the baryon density, Ωb

• D abundance is the most sensitive to changes Ωb.

• Comparing primordial light element abundances can test BBN (e.g. number of neutrino species).

• The Ωb values predicted by He, D and Li do not agree!

“believed to be theprimordial value…”

D/H = 3.3 +/- 0.3 x 10-5

~150 km/s

The high redshift deuterium abundance D/H

Current status of D/H

• We have made a new D/H measurement which is smaller than the cosmic background radiation D/H prediction.

• Is the D/H scatter real? If so, is this a scatter in the primordial D/H, or evidence of D evolution?

Physics as we currently have it

SOME SIMPLE ASSUMPTIONS(isotropy, homogeneity, laws of physics constant)

THE STANDARD MODEL (and unhappy cosmologists)

+

SOME ADD-ONS(eg inflation)

BLISSFULLY HAPPY COSMOLOGISTS

BUT: we can only directly detect 4% of the matter in the Universe. The rests just “gravitates”.

=

+ =

On a large enough scale, the Universe appears the same in all directions at a given time, from anywhere “THE COMOLOGICAL PRINCIPLE” ISOTROPY: The Universe is the same in all

directions from any point

HOMOGENEITY: We do not occupy a “special” position in the Universe. Therefore, the Universe looks “the same” to any observer, anywhere, at any timeThe Big Bang model is based on the Cosmological Principle. The assumptions: (1) make the calculations easier, and (2) are justified (partly) by the CMB

Time

Big bang(T=0)

Space-time diagram

Us, now

Cosmic microwave background radiation(T~300,000 years)

13.7 By

The main point of this diagram:

if >8 deg (approx), regions further apart than A and B have communication times longer than the age of the universe at that time. Therefore, why is T(CMB) so uniform?

The standard big bang model without inflation or some other “add-on” provides no explanation.

The horizon problem

Hot topic #1:The accelerating universe

The “scale factor”. We cannot observe this directly.We can only infer it from incomplete and biased observations.

Distance modulus and acceleration – 73% of the universe is missing (and, worse (?), is anti-gravity)

• Why our particular set of (numerical values for) the fundamental constants (given the system of units)? Anthropic explanation? Should we be able to explain the numerical values from first principles?

• Fine tuning: Eg, electromagnetic coupling constant, = 1/137. Slight decrease: molecular bonds break at lower T, number of stable elements in periodic table increases. A 4% change would shut down stellar heavy element production – no life. Many other examples.

• Were our present-day constants laid down a the beginning or have they evolved into a more finely-tuned set?

• What is a fundamental constant? We formulate the “physical laws” in terms of observables, which = measurements wrt to some standard, but is that fundamental or merely “human”?

• Dimensional vs dimensionless. Does it make any sense to generate theories in which, eg, c varies? What changes can we actually look for?

• Should (can?) all physical laws be satisfactorily re-expressed in terms of dimensionless constants?

Hot topic #2: “Fundamental” constants

History: Dirac, Milne, 1937. The first to ask “Do the constants of Nature vary?”

Theoretical motivation: unification models – more dimensions (“compactified”). Cosmological evolution? String theories generically allow for variations.

Dimensionless ratios – the things we can actually check

To Earth

CIVSiIVCIISiII

Lyem

Ly forest

Lyman limit Ly

NVem

SiIVem

CIVem

Lyem

Ly SiII

quasar

Quasars: physics laboratories in the early universe

c

e

2

Highly exaggerated illustration of how transitions shift in different directions by different amounts – unique pattern

Relativistic shift of the central line in the multiplet

Procedure1. Compare heavy (Z~30) and light (Z<10) atoms, OR

2. Compare s p and d p transitions in heavy atoms.

Shifts can be of opposite sign.

1qEE2

0

z0zz

Ez=0 is the laboratory frequency. 2nd term is non-zero only if has changed. q is derived from relativistic many-body calculations.

)S.L(KQq K is the spin-orbit splitting parameter. Q ~ 10K

Numerical examples:

Z=26 (s p) FeII 2383A: = 38458.987(2) + 1449x

Z=12 (s p) MgII 2796A: = 35669.298(2) + 120x

Z=24 (d p) CrII 2066A: = 48398.666(2) - 1267xwhere x = z02 - 1 MgII “anchor”

Murphy, Webb, Flambaum, MNRAS, 345, 609, 2003

Current status of alpha

The bottom line:

: current best data suggests a deviation from present day value, but large statistical result is in preparation

Hot topic #3:Something weird happens to the Universe at z=3

Spectroscopic coverage by data release in equatorial co-ordinates

DR5 spectroscopic coverage in Galactic co-ordinates

Spectra of 19 quasars with 5.74 <z <6.42 from the SDSS, (Fan et al. 2005). Some show no transmitted flux shortward of Lyα emission (“Gunn-Peterson trough”), indicating a non-negligible neutral H fraction in the IGM.

o 11,200 quasars with emission redshifts > 2.3o Broad absorption line quasarso Damped Lyman alpha absorberso Some M starso Probably some other contaminants

How much information does a quasar sample like DR5 contain?

For z>2.3, forest spans 3.3x(1215-1025)=627A.Spectral sampling is ~2A/pixel

There are therefore 11200 x 627/2 = 3.5 million independent estimates of cosmological hydrogen opacity, distributed across 11200 independent lines of sight through the universe

H0=70 km/s/Mpc, m=0.25, =0.75

Lyman alpha forest DA measurement region

Solid vertical lines – Lyman alpha and beta wavelengthsDot-dash vertical lines – Bernardi et al fitting regionShaded vertical lines – our fitting region

Vanden Berk et al, Astron.J., 122, 549, 20012200 quasars. 80 emission features identified

Regions selected (by us) for estimating the underlying power law:{1268.0,1292.0}, {1312.0,1328.0}, {1345.0,1365.0}, {1430.0,1475.0}, {1680.0,1700.0}, {1968.0,1982.0}, {2020.0,2040.0}, {2150.0,2170.0}, {2190.0,2250.0}

Composite quasar spectrum from SDSS

Left panel: Regions with large-scale overdensities form large concentrations of galaxies (dots) whose ionizing photons produce enormous joint ionized bubbles (upper left). At the same time, galaxies are rare within large-scale voids, in which the IGM is still mostly neutral (lower right).

The spatial structure of cosmic reionization

Right panel: A numerical simulation of reionization (Mellema et al. 2006) indeed displays such variation in the sizes of ionized bubbles (orange), shown overlaid on the density distribution (green).

Loeb astro-ph/0711.3463

4He nuclei are formed from 2 neutrons and 2 protons

Neutron decay results in Nn/Np ~ 1/7

Total number of particle nuclei available is N = Nn + Np = 8Np/7

Total number of particles bound up in 4He nuclei is therefore equal to all of the neutrons (the H nucleus has none) plus an equal number of protons, i.e. Nn + (Np/7) = 2Np/7

Therefore, 4He nuclei contribute (2Np/7)/(8Np/7) or ~25% of the total mass of the universe.

Why helium is cosmological in origin

Redshift

DR5 and HST points set to same normalisation (somewhat arbitrary)

Interpretation

6300 quasar spectra from DR5 show an abrupt change in the effective optical depth of the cosmological distribution of neutral hydrogen. It’s likely this is the discovery of helium reionisation caused by an increase in the background UV radiation field.

Quantitative modelling of this information has yet to be explored in detail. It is likely that the upshot of such detailed measurements will be strongent constraints on a number of important cosmological parameters, especially the UV background flux (with corresponding interesting constraints on sources of this radiation.

Concluding remarks

1. In general terms, BBN is fantastically successful, over almost 12 orders of magnitude in relative abundances.

2. There is also remarkable consistency between CMB measurements and other independent measurements.

3. Hot topic #1: On the other hand, the price we pay for accepting GR+cosmological principle+”add-ons” is 73% dark energy (which we have absolutely no idea about), 23% dark matter (which we also don’t yet understand). Obviously there is some fundamental gap in our knowledge, either of gravity itself, or of what the universe comprises, or both.

4. Hot topic #2: Understanding such fundamental issues as the variation (or lack thereof) of the fundamental constants may shed light on the problems above.

5. Hot topic #3: At z=3, the reionisation of helum appears to begin. This has not been detected before but is expected theoretically. Measuring the details of this phenomenon will provide fundamental information about the first generation of stars and galaxy evolution.