Big Bang ExpansionA textbook example of how observations drive theory

Andreas KornInstitutionen för fysik och astronomi, Uppsala universitet

The evolution of the Cosmos

Big Bang Big-Bang nucleosynthesis

microwave background

formation of galaxies

formation of the Sun

astronomerson Earth

t0 3 min 380 000 yr 200 Myr 9 Gyr 13.7 Gyr

Using a variety of techniques, we can, in principle, study all phases of cosmic

evolution

The pillars of the Big-Bang theory

I. The expansion of the UniverseDistant objects are receeding following a simple v = H0 d law with H0 ¼ 70 km/s/Mpc

II. The Cosmic Microwave Background (CMB)The afterglow of the Big Bang emitted when the Universe became transparent to photons (380 000 yr after the Big Bang)

III. Big-Bang Nucelosynthesis (D, He, Li)The observed abundances of light elements in the oldest objects can be explained by fusion reactions shortly after the Big Bang

IV. Component ages All objects we can date accurately indicate a finite period of structure formation

Accumulating evidence…

I. "Hubble" expansion(1920s, Hubble, but also Knut Luntmark och George Lemaître)

II. Cosmic 3K microwave background(predicted in the 1940s, serendipitously discovered 1965, detailed studies since the late 1980s)

III. Big-Bang Nucleosynthesis(theory developed in the 1940s)

He / Li / D in old objects(observations since the 1970s)

IV. (I am working on it...)

I. Theorising about an expanding Universe

Einstein introduced Λ (the cosmological constant) to make his field equations return a static solution. Anything but this was unthinkable for him (1910s).

It was Alexander Friedman and Georges Lemaître who realized in the 1920 that expanding (or contracting) solutions are possible. Lemaître even interpreted the existing observational data as evidence in favour of cosmic expansion (1927), before Hubble (1929).

I. Observing the expanding Universe

Hubble’s original diagram (1929):

H0 = 500 km/s/Mpc

I. Observing the expanding Universe

Since the late 1990s:

II. What is the CMB?

1. The afterglow of the primordial fireball emitted when the Universe had cooled enough for electrons to combine with protons (to form H atoms).

2. A record of the conditions in the Universe 380 000 yr (time of last scattering) after the Big Bang (T ≈ 4000 K).

What was the Universe like in those days?

As we shall see, it was largely uniform (confirming the homogeneity and isotropy assumption): temperature (and thus) density fluctuations were confined to 1 part in 105.

Matter (m) tries to collapse gravitationally, but photons have thus far prevented more than the formation of structure seeds. The cosmic web will later form out of these sedes.

II. Observing the CMB

Variations on the 100 μK scale

BOOMERanG

WMAP

II. CMBology (Era of Precision Cosmology)

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

(CMB movies)

ΛCDM prediction

II. CMBology (Era of Precision Cosmology)

Ωtot=1 constrain Ωb

acoustic oscillations

II. CMBology (Era of Precision Cosmology)

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III. Big-Bang Nucleosynthesis

Three minutes after the Big Bang, the universum has the right temperature (¼ 109 K) to act as a as fusion reactor.

The expansion continues and the neutrons decay, thus quenching the fusion after a few minutes.According computation predict highly accurate abundance ratios of hydrogen, helium and lithium as a function of the matter density B.

III. Observing BBN

since the1970s: helium in star-forming regions of primeval galaxies

1980s:

lithium in old stars in the halo

1990s:

deuterium in towards high-redshift quasars

B is predicted to be 1-2% of the critical matter/energy density required to close the Universe.

III. The post-WMAP picture of BBN

WMAP: b / c = (4.5 § 0.1) %

somewhat more helium than measured,

somewhat less deuterium, (surprisingly large observed scatter)

markedly more lithium than measured.

WMAP

WMAP

tot = 1

Dark matter: 23 %

Dark energy (): 73 %

Big Bang: a self-consistent theory?

Big Bang Expansion is a vastly successful theory. However, since the late 1990s it has led us to realize that we practically know nothing about 95% of the Universe’s content.

The need for Dark Matter may be reminiscent of the etherdiscussion in the latter half of the 19th century. However, no other theory (e.g. of MOND-type) has been able to explain a variety of observables equally well. Indications for dark-matter candidates may come from the Large Hadron Collider soon.

Dark Energy is even more enigmatic. It will likely take decades to unravel its nature, if it really exists.

optical / X-ray / DM

Other possible variants

There could be/have been more neutrino families which would alter BBN and CMB. But it seems as if Nν is close to 3.

There could have been (super-symmetric)particles present during BBN whose decay would alter BBN prediction. This may solve the lithium problem

(cf Rickard’s seminar).

ΛCDM while being highly successful in describing the Universe’s large-scale structure still faces a number of more local tests. We do not know whether the Milky Way fits the big picture.

Summary

The realisation that the Universe we live in is expanding came about in the 1920s as a solution to Einstein’s field equations supported by observations.

Subsequent work led to the idea of an early hot and dense phase (BB) with properties appropriate for the creation of a) an afterglow (CMB) and b) chemical elements (BBN). Both predictions have been tested observationally, and confirm the BB expansion theory, partly in glorious, epoch-shaping detail (WMAP).

Dark Energy (making up >70% of Ωtot) remains a mystery.

Literature

Hubble E. 1929, "A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae" Proceedings of the National Academy of Sciences of the United States of America, 15 (3): 168

Friedman A. 1922, "Über die Krümmung des Raumes", Zeitschrift für Physik 10 (1): 377

Lemaître G. 1927, "Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques", Annales de la Société Scientifique de Bruxelles, 47: 49

Penzias A.A., Wilson R.W. (1965), "A Measurement Of Excess Antenna Temperature At 4080 Mc/s", Astrophysical Journal Letters 142: 419

Walker T.P., Steigman G., Kang H.-S., Schramm D.M., Olive K.A. 1991, "Primordial nucleosynthesis redux", Astrophysical Journal 376: 51

Larson D. et al. 2010, "Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Power Spectra and WMAP-Derived Parameters“ http://arxiv.org/abs/1001.4635

Uppsala’s contribution (not quite in the same league):

Korn A.J. et al. 2006, "A probable stellar solution to the cosmological lithium discrepancy", Nature 442: 657

http://arxiv.org/abs/1001.4635