COSMIC MICROWAVE RADIATION: THE FOOTPRINT OF THE “BIG BANG”
In our previous videos on dark energy and dark matter, we’ve shown that only a tiny 5% of the
energy density of our universe is made up of the ordinary matter we know on earth.
The vast majority of the universe in fact is dark matter and dark energy,
two entities which at the present state of knowledge, we are unable in any way
to reveal directly, nor even to perceive. It's shocking having to admit that nowadays we
don’t know what 95% of our universe is, isn't it? However, how can we get these estimates? How
can cosmology investigate the universe at such large scales? In the last decades, astronomers
have discovered a fundamental element to study our universe: cosmic microwave radiation (CMB).
What the heck is cosmic microwave radiation? Where is it? How was
it discovered and when? What is it made of? This and all other questions we will answer
during this video... 1 - STORY
When general relativity was published, Einstein believed the universe as a steady system.
However, very early, plenty of observations showed that the universe was not only expanding
but was even accelerating its expansion rate. In 1931 the Belgian physicist Georges Lemaître
went beyond and suggested that if the universe was not a static system but was expanding,
then by traveling back in time it could be traced back to the moment of its hypothetical birth.
He published these ideas in 1927, in the paper “ The primordial atom hypothesis”
Lemaître’s theory was named "Big Bang Theory” by the astronomer Fred Hoyle,
in 1949, during a BBC radio broadcast.
Big Bang theory was endorsed and developed by the physicist George Gamow,
co-author in 1948, together with Ralph Alpher, of a paper about primordial nucleosynthesis.
In the wake of this work, the same year, George Gamow, together with physicists Alpher and Herman,
predicted radiation called “Cosmic Background Radiation”: in other words, they argued that
if Big Bang theory was right, expansion of the universe would have necessarily left the footprint
of the very early universe into a microwave region of the electromagnetic spectrum.
They predict the so-called "cosmic microwave background”(CMB), the "echo"
of the Big Bang, the footprint of our universe. However, it took another 15 years to detect it.
As often happens in science in fact, many discoveries that will mark the
future of later science, happen by chance, almost incidentally. when this happens,
it is said that everything happens out of serendipity. And this was also the case…
In 1964, at “Bell laboratories” in Crawford Hill - New Jersey,
two steps from the University of Princeton, the physicists
Arno Penzias and Robert Wilson were studying some radio emissions from the Milky way using
a "Holmdel Horn Antenna". This is a large microwave horn antenna,
constructed in 1959 and previously used as a satellite communication antenna
and radio telescope by NASA. Penzias and Wilson were
calibrating a reception system to centimeter wavelengths when detected a background noise.
At first, thinking of some interference, they tried to eliminate any source of the
disturbance. Nevertheless, the signal remained steady with the same intensity. This was an
omnidirectional background signal, not associated with any star, galaxy, or other celestial object.
A faint background noise, or glow, strongest in the microwave region of the radio spectrum.
It didn’t even depend on the direction of the antenna.
They detected the cosmic microwave radiation that George Gamow,
Ralph Alpher, and Robert Herman had predicted.
In 1978, Penzias and Wilson were awarded the Nobel Prize in physics for this discovery.
2 - WHAT’S CMB AND WHY IS SO IMPORTANT?
Let's go a little bit more into detail and let's try to figure out what exactly is CMB.
This is radiation, which permeates almost uniformly the cosmos.
It is deemed to be the remnant from the early stage of the universe:
that’s why it’s also called “relic radiation”: it’s a very important source of information.
At first approximation, CMB is isotropic and homogeneous:
it’s the same everywhere in the universe and in every direction.
CMB has a perfect black body spectrum: the most perfect black body ever measured in nature.
As a matter of fact, it has a nearly ideal Planck spectrum at a temperature
of about 2.7 K and according to Wien’s law, has a peak in the microwave wavelength.
A black body is an ideal object in thermodynamic balance at a certain temperature,
which can absorb and emit all the electromagnetic radiation that receives. This is very important
because indicates that, when CMB was emitted, the universe was in thermodynamic equilibrium.
CMB spectrum shows a specific pattern that fits very well a very hot gas pattern,
expanding uniformly within the actual dimensions of the universe.
CMB spectrum analysis has proven to be a mine of information about
the early stage and the evolution of the universe.
According to CMB lambda model (the most upgraded cosmological model),
cosmic background radiation was born in a very precise moment in the evolution of the universe:
the “recombination era”, followed by the so-called “decoupling matter-radiation”, about 370ky ago.
At this stage, the universe looks like an incredibly hot ionized gas (plasma) in expansion,
made up of atomic nuclei of Deuterium, Helium, Lithium, surrounded by electrons, charged
particles, and photons, in a “primordial soup” in thermodynamic balance. In these extreme conditions
(temperature now is about 109 K) binding of electrons with atomic nuclei cannot occur.
Moreover, photons are continuously deflected and cannot travel significant distances without
interacting with any particle. So, radiation is somehow trapped in the plasma, and the universe
in this phase is “foggy”. Let’s imagine a little gate of a church seen from outside: if it’s tiny
enough, it appears dark, even if the church is bright inside, because light cannot escape.
As the universe continues to expand, it cools down until 4000K,
when finally free electrons can bind with hydrogen and helium nuclei, and form neutral atoms.
The “recombination era” begins 370ky ago and lasts for about 100 ky.
During this phase, the universe becomes more and more transparent to radiation,
as photons no longer held, can get away and begin to travel along with space:
this moment is THE DECOUPLING of radiation from matter. CMB originate at this time.
CMB detected nowadays, comes from a spherical surface called the “surface of last scattering”.
This surface represents the set of the point in space-time where decoupling happened.
Therefore CMB is considered the most ancient image of the universe obtainable:
before the last scattering, the universe was opaque and
is impossible to receive any photon coming from a period prior to this event.
The photons released during the decoupling era originally had a temperature/energy
of around 4000 K. These would have been a visible yellow/orange or white light. Since the universe
has expanded since that moment, over billions of years, CMB has now been red-shifted from
visible light to radio waves (microwave radiation corresponding to a temperature of about 2.7 K).
Those same photons can still be detected as radio waves today and form the cosmic microwave
background.and they provide crucial evidence of the early universe and how it developed.
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3 - CMB EXPERIMENTS
Experiments and measurements over time have deeply investigated CMB, which has proven to
be a veritable mine of information, through which our understanding of cosmic scales has made leaps
and bounds, allowing us to deduce much data about the deep structure of the universe.
COBE - Cosmic Background Explorer satellite (from 1988 to 1993)
The first big project for measuring CMB was the Cosmic Background Explorer satellite a.k.a COBE.
COBE was sent into orbit from 1988 to 1993 and had on board three main instruments: DIRBE,
FIRAS, and the DMR.
FIRAS: Far-InfraRed Absolute Spectrophotometer was
designed to measure the spectrum of CMB everywhere in the universe.
DIRBE: Diffuse InfraRed Background Experiment was designed to gathered data in the infrared band.
DMR: Differential Microwave Radiometer: a microwave instrument to map variations in the CMB.
For the first time, COBE measured the CMB spectrum with great precision.
FIRAS, gathered data for a period of more than 10 months,
confirming that CMB has a perfect black body spectrum with a temperature of 2.73 K.
Furthermore, DMR detected some extremely faint temperature fluctuations, One part in
100.000 only. So, throughout the universe, there are colder and warmer areas of CMB.
These variations are called anisotropies and represent a fundamental source of data.
First of all, they constitute overwhelming evidence supporting the big bang as a cosmological
theory: even if different processes can produce a black body spectrum, no other model different from
Big Bang has been capable to explain anisotropies That’s why CMB together with Hubble's law are
regarded as the best available evidence for Big Bang theory.
The analysis of CMB and its anisotropy not only allowed to reconstruct of the processes
that occurred in the universe during the era of recombination and decoupling, when it has been
emitted but made also possible to obtain important information on the structure and composition of
the universe on global scale, by examining the effects and interactions it undergoes
during the intergalactic journey from the surface of the last scattering to reach up to us.
The features of the universe on a global scale are described in
theoretically models by cosmological parameters. In this sense, CMB has been and still is a crucial
tool for estimate cosmological parameters and elaborating the cosmological model of Big Bang.
COBE was a very successful experiment and George Smoot and John Mather, respectively
head of DMR and FIRAS projects, were honoured by the Nobel Prize in Physics in 2006 for their work.
During the next decade, anisotropies were further studied by a large number of probes
(either on the ground or through balloons). In 2000-2001 experiments (including the
most important was “Boomerang”), measuring the angular width typical of anisotropies,
together with other cosmological data, these experiments even get to assess
that the geometry of the universe finding that it has an approximately flat geometry.
WMAP (from 2001 to 2010)
Originally known as the Microwave Anisotropy Probe (MAP), this uncrewed spacecraft launched
in 2001 from Florida, was renamed WMAP in honour of cosmologist David Wilkinson (after his death
in 2002). The WMAP objective was to better detect the temperature differences in CMB
and to use them to measure the universe's geometry, content, and evolution.
WMAP's measurements played a key role in developing, refining and testing
the current Standard Model of Cosmology: the Lambda-CDM model. Thanks to the measurements
carried out, the precision and accuracy of the cosmological parameters were further improved.
In October 2010, WMAP spacecraft ended its mission and was substituted by Planck SATELLITE.
PLANCK (from 2009 to 2013)
Planck has been a space observatory built by the European Space Agency (ESA)
from 2009 to 2013, which mapped the anisotropies of CMB at microwave and infra-red frequencies,
with high sensitivity and small angular resolution. Planck had in fact a higher
resolution and sensitivity than WMAP: it observed in 9 frequency bands rather than WMAP's 5,
allowing improved astrophysical foreground models. Planck has nowadays defined the most precise
measurements of several key cosmological parameters, including the average density
of ordinary matter and dark matter in the Universe and the age of the universe.
It was renamed in honor of the German physicist Max Planck (1858–1947), who
derived the formula for black-body radiation. Planck was launched in May 2009 AND measured
the CMB from 2009 to 2013 and aims to refine the measurements made by WMAP.
On 2013, the European-led research team behind the Planck cosmology probe
released the mission's all-sky map of the cosmic microwave background
but the final papers by the Planck team were released in July 2018. These are
the most recent and accurate cosmological parameter measurements currently available
The map suggests the universe is slightly older than previously thought.
According to the map, subtle fluctuations in temperature were imprinted on the deep sky
when the cosmos was about 370,000 years old. According to the team, the Universe is about
13.79 years old and is approximately flat. It contains about the 5% of ordinary matter,
about the 26 % of dark matter and about 69 % of dark energy.