A universe in constant evolution - Leonardo Fernández Jambrina

9.8

A universe in constant evolution.

The Lives of the Stars.

Free download of the chapter in PDF

Music credits

"There are more things in heaven and Earth, Horatio, than are dreamt of in your philosophy", 
WILLIAM SHAKESPEARE The tragedy of Hamlet, prince of Denmark, ¿1599-1601?

"Any great flash of understanding is only half completed in the illumined circle of the conscious mind; the other half takes place in the dark loam of our innermost being. It is primarily a state of the soul, and uppermost, as it were at the extrem tip of it, there the thought is poised -like a flower.", 
ROBERT MUSIL The confusions of young Törless, 1906.

My own personal remembrance of Cosmos is a summer in Asturias, watching the series after dinner in a strand restaurant in my dear town of Llanes. I quite remember the city of Vinci (I cannot understand why), as an example of travelling at the speed of light, with several zeros less, so that it could be experienced by a cyclist. What I could not foretell is that ten years later I would be working in General Relativity.

The idea we had of the universe in the 80's, basically the one which prevailed since the 20's, was that of a homogeneous universe at large scale, which, depending on the amount of matter in it, could expand forever or recontract to a new singularity, counterpart of the initial Big Bang.

At that time observations favoured a universe close to the divide between those two possibilities. This raised the problem of "dark matter", since the amount of observed matter was by far much lower than such limit. There should be hidden matter in the form of dead stars, unobserved planets, massive neutrinos... By the way, these discussions are still alive.

The change of paradigm arose at the end of the century, when observations based on supernovae Ia suggested that our universe was expanding, but in an accelerated fashion [1], fact that did not match the models I have just mentioned.

Since then this fact has been supported by other kinds of observations, entangling even more the problem of our lack of knowledge about the universe. According to satellite Planck observations [2], about 4.9% of the content of the universe is deemed to be ordinary matter, 26.8% would be dark matter and 68.3% would be the "dark energy" that triggers the accelerated expansion of the universe. ¡It is a lesson in humility to acknowledge that we just take account of  5% of our universe!

We shall have to wait at least until next update of this conundrum, since we do not have so far the final answer. There are scientists who put forward a modification of the theory of gravity, beyond General Relativity, but there are also others who keep the theory, but adding a new field which would take account of dark energy.

Expanding Universe. 

Public Domain. Credit: NASA / STSci / Ann Feild

https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/

At the time being, the simplest model that answers most questions is LCDM: this model suggests a spatially flat universe within the theory of General Relativity, in which dark matter is "cold" (CDM stands for  Cold Dark Matter), in the sense that it moves much slower than the speed of light, and dark energy is described by a cosmological constant L [3]. This is a nice example of revenge of history: Einstein included the cosmological constant in order to keep the universe stationary, until he had to admit the expanding universe and to acknowledge that the cosmological constant had been his greatest blunder. It seems that the cosmological constant refused to fade away, as quantum field theories pointed out, since they predicted large cosmological constants.

Other proposals substitute the cosmological constant for a field, such as "quintessence" (physics have a kind of sense of humour on choosing names) [4]. It is not a minor issue, since, depending on the nature of this field, the universe might not expand forever, as one could expect, but end up in a Big Rip [5], another type of singularity, different from the initial one, at which all structures, galaxies, solar systems... would tear away on approaching the final time.

But we are becoming quite apocalyptic. Let's go on with the good news instead. Beyond the cosmological constant, one of the most outstanding predictions of the theory of General Relativity was the existence of gravitational waves.

Until 19th century we knew about material waves (waves on the surface of water, sound through air, seismic waves on Earth's crust...). But Maxwell's laws of electromagnetism challenged our ideas about waves, since they described a new type, electromagnetic waves, which propagate in vacuum without any material support. This aroused much controversy at the moment, invoking the existence (we have heard this story before!) of a fluid, ether, which permeated the universe. But nowadays propagation of light, radio waves, X rays, microwaves in vacuum is fully accepted.

But General Relativity was one step ahead, since one of its consequences was that the very spacetime could fluctuate, giving rise to gravitational waves.

Why gravitational waves had not been detected? Due to their extreme weakness. At the scale of protons, the gravitational force is 10^-36 times weaker than the electromagnetic force. Not only that, electromagnetic radiation is fundamentally dipolar (the simplest antenna is a dipole, formed by an oscillating pair of a positive and a negative charge). But in gravitation we have no negative masses, so we have to go down one order to look for quadrupolar radiation (due to non-spherical objects of varying density). This means that gravitational radiation dilutes with distance much quicker than electromagnetic radiation. And this, together with its weakness, has been a pain in the neck for its detection [6].

Due to the weakness of the gravitational force, we cannot expect emission from an antenna, such as the usual radio ones, in experiments on the Earth's surface. We are forced to wait for very energetic phenomena, though distant, such as supernova explosions, collapse of binary systems formed by black holes or neutron stars rotating at high speed around their centre of mass. As gravitational waves alter spacetime, it was expected that highly precise measures of distances could detect the influence of such waves.

In fact, although they had not been directly detected, we did have indirect evidence of their existence: in 1974 Hulse and Taylor detected radio waves coming from a binary pulsar, PSR B1913+16, formed by two neutron stars rotating around their centre of mass with shorter and shorter period and orbit. This meant energy loss, which could be explained with outstanding precision by taking into account the energy of the gravitational waves emitted, though not detected, by the system [7]. Therefore, for final confirmation of the existence of gravitational waves only direct detection was missing.

It is easy to understand how a detector works, since the principle is the same as in laser interferometry: a laser beam is split in two with orthogonal directions, which are brought together again by mirrors, allowing for observation of interference patterns. The idea behind is that the arrival of gravitational waves from space would cause a distortion of spacetime which would shorten or stretch the path of the beams of the interferometer, altering the interference pattern. At LIGO (Laser Interferometer Gravitational Wave Observatory), the interferometer arms are four kilometres long and allow detection of variations about 10^-18 metres. In order to avoid spurious observations and to increase the possibilities of detection, these laboratories are located in distant places to prevent local sources of error.

On 14th September 2015 the signal of the collapse of a system formed by two black holes was detected for the first time [8]. Since then, between LIGO and Virgo (a similar European laboratory located in Italy) the number of detections of gravitational waves has increased, issued by binary sytems of black holes, neutron stars and mixed ones. The age of gravitational wave astronomy has begun!

Looking ahead, detection is planned to be carried out in space, thanks to  projects like LISA (Laser Interferometer Space Antenna), with three satellites linked by laser beams.

Although both phenomena seem to outshine everything else, I would not feel comfortable finishing my account without mentioning a third one, the oldest one: observation of inhomogeneities in cosmic background radiation.

At the beginning, the universe was a plasma consisting of electrons, protons, neutrons and photons, which could not form atoms due to its high energy. Only when its temperature dropped below 3000 K, when the universe was 380000 years old, photons stopped ionizing forming atoms and light could travel without being dispersed. Due to the expansion of the universe, the temperature of those photons is 2.7 K nowadays [9]. This radiation was detected by chance for the first time in 1965 by Penzias and Wilson [10], since it came up as a background noise in their experiments. In fact, the "snow" we used to observe in our old analogue TV sets was partially due to reception of background radiation!

At that time the detection of cosmic microwave background (CMB) radiation was considered an evidence for the Big Bang theory against other models such as the steady state theory. However, its success posed some problems, as the radiation was highly isotropic, though it should allow some inhomogeneity in orde to take account of the irregular distribution of matter nowadays. In 1992 the COBE satellite observed such inhomogeneities for the first time [11]. Other space missions, such as WMAP and Planck, as well as other observations on Earth, have completed the inhomogeneity map, both in intensity and polarization, of cosmic background radiation, providing evidence for instance for a inflationary age at the beginning of the universe.

As a personal curiosity, the publication of COBE results nearly matched the publication of my first paper on non-singular cosmology. As a disclaimer I must say that the role of the initial singularity is far from being clear. It is something that only a quantum theory of gravity would envail. But, who knows? Maybe for the next update of this book!

Referencias:

[1] A. Riess et al, 1998, Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant, Astronomical J. 116, 1009.
[2] N. Aghanim et al, 2020, Planck 2018 results. VI. Cosmological parameters, A & A (en imprenta).

[3] S. Carroll, 2001, The Cosmological Constant, Living Reviews in Relativity 4: 1. 

[4] R.R. Caldwell, R. Dave, P.J. Steinhardt, 1998), Cosmological Imprint of an Energy Component with General Equation-of-State. Phys. Rev. Lett. 80, 1582–1585.

[5] R.R. Caldwell, M. Kamionkowski, N.N. Weinberg, 2003, Phantom Energy: Dark Energy with w<−1 Causes a Cosmic Doomsday, Phys. Rev. Lett. 91, 071301 (2003).

[6] C.W. Misner, K.S. Thorne, J.A. Wheeler, 1973, Gravitation, San Francisco, W. H. Freeman.

[7] J.M. Weisberg, J.H.Taylor, L.A. Fowler, 1981 Gravitational waves from an orbiting pulsar, Scientific American 245, 74–82. 

[8] B.P. Abbott et al, 2016, Observation of Gravitational Waves from a Binary Black Hole Merger, Phys. Rev. Lett. 116, 061102.

[9] R.A. Alpher, R. Herman, G.A. Gamow, 1948, Thermonuclear Reactions in the Expanding Universe, Physical Review 74: 1198.

[10] A.A. Penzias, R.W. Wilson, 1965, A Measurement of Excess Antenna Temperature at 4080 Mc/s, Astrophysical Journal 142, 419.

[11] N.W. Boggess et al, 1992, The COBE Mission: Its Design and Performance Two Years after Launch, Astrophysical Journal 397, 420.

Leonardo Fernández Jambrina.

Doctor en Ciencias Físicas, Licenciado en Ciencias Físicas y en Ciencias Matemáticas.

Catedrático de Matemática Aplicada, Universidad Politécnica de Madrid.