A universe in
constant evolution.
The Lives of the Stars.
"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?
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.
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
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).
[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.
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