Planck’s Time and the “Oldest Light” in the Cosmos

A photomontage showing an image of the Planck spacecraft superimposed on a sky planisphere of the Cosmic Microwave Background $ ($CMB$ )$ Radiation.Who, What, Where?

What happened at time t = 0?  is still anybody’s guess.  At least, earlier observations of Planck’s radiation had suggested the first generation of stars were bursting into life by about 420 million years after the Big Bang.  However, scientists from Europe’s Planck satellite mission now say the first stars lit up the Universe later than was previously thought…

The Planck time is the time it would take a photon moving at the speed of light to travel across a distance equal to the Planck length – the scale at which classical ideas about gravity and space-time cease to be valid, and quantum effects dominate; the ‘quantum of length’; the smallest spatial measurement with any meaning, roughly equal to 1.6 x 10-35 m – about 10-20 times the size of a proton.

Anyway, the Planck time is the ‘quantum of time’, the smallest measurement of time that has any meaning, and is equal to t = 10-43 seconds.  No smaller division of time has any meaning.  Within the framework of the current laws of physics, we can say only that the Universe came into existence when it already had an age of 10-43 seconds.

General Relativity breaks down at the Planck time t = 10-43 s.  So, progressing back from that point on to earlier times requires a theory of Quantum Gravity – a field of passionate cutting-edge physical research.  Inflation remains without a firm grounding until this very early era is better understood.  And however limited progress has been made with quantum cosmology, the new all-encompassing M-theory offers several intriguing lines of enquiry.

A diagram showing how the four fundamental forces - the strong force, the electromagnetic force, the weak force and the force of gravity - were one single unified force at the very beginning of the Universe.
A Theory of Quantum Gravity would unify General Relativity with Quantum Physics.

What is becoming clear from the Planck investigation/survey is that the simplest models for how the super-rapid expansion of the Universe might have worked may no longer be tenable, suggesting some exotic physics will eventually be needed to explain it.

Professor George Efstathiou and his team of the Planck Science Collaboration have made the most precise map of the “oldest light” in the cosmos.

Planck’s new data now indicates this great ignition was well established by some 560 million years after it all got going.



Although a difference of 140 million years later might not seem very significant in the context of the 13.8-billion-year history of the cosmos, it is actually a very big change in our understanding of how certain key events progressed at the earliest epochs.

The assessment is based on studies of the “afterglow” of the “Big Bang”, the ancient light called the Cosmic Microwave Background (CMB), which has been washing over the Earth since the beginning of the Universe.


The Cosmic Microwave Background Radiation


A photograph of Arno Penzias and Robert Wilson in front of the horn antenna at Bell laboratories in 1993.
Arno Penzias and Robert Wilson pose in 1993 with the horn antenna they used to discover the cosmic background “fossil” radiation at Bell labs.

The accidental discovery of the Cosmic Microwave Background (CMB) in 1964 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated by the pair in the 1940s, and earned the discoverers the 1978 Nobel Prize.

The CMB is the thermal radiation left over from the “Big Bang” of cosmology .  Fundamental to astrophysics, the CMB is the oldest light in the Universe, dating back to the epoch of recombination.

With a traditional optical telescope, the space between stars and galaxies (or background) appears completely dark.  However, a sufficiently sensitive radio telescope registers a faint background glow, almost exactly similar in all directions, that is not associated with any star, galaxy, or any other object.  This glow is strongest in the microwave region of the radio spectrum.

The photons that existed at the time of photon decoupling have been propagating ever since, though their light has been growing fainter and less energetic, since the expansion of space causes their wavelength to increase over time (and wavelength is inversely proportional to energy according to Planck’s relation).

A diagram explaining the surface of last scattering of the Cosmic Microwave Background from the point of view of an observer on Earth.Under the “Big Bang” model of cosmology, before redshift z = 1000, photons in the CMB were hotter by a factor of (1 + z), and hence able to ionize the hydrogen in the Universe.  Compton scattering of photons and electrons tightly couples the two species.  Meanwhile, electromagnetic interactions couple the electrons and baryons together.  The photon-electron-baryon system thus can be thought of as a single tightly coupled fluid,called the photon-baryon fluid to identify the dominant dynamical components.

After the “recombination” epoch at z = 1000 when neutral hydrogen forms, CMB photons mainly just stream toward the observer at the present.  Hence temperature differences on this ‘surface of last scattering’ become the anisotropies in the Cosmic Microwave Background temperature that we observe today.


Mapping the CMB

A picture of Planck's anomalous sky, the hemispheric asymmetry and the "cold spot".
Planck’s anomalous sky: the hemispheric asymmetry and the cold spot  Source: ESA/NASA

The European Space Agency’s (ESA) Planck satellite mapped this “fossil” light between 2009 and 2013.  The gathered data  contains a wealth of information about the conditions in the early Universe, and can even be used to work out its age, shape and do an inventory of its contents.  Scientists can also probe it for very subtle “distortions” that tell them about any interactions the CMB has had on its way to us.

The map was produced using the data from the European Space Agency’s Planck probe-scope positioned at the L2 Earth/Sun Lagrange point – 1.5 million kilometres away in the vast depths of space.  Planck scanned the entire sky, and advanced boffinry was used in order to purge its imagery of light emitted by such stuff as distant stars or galaxies.

One of the distortions or anisotropies would have been imprinted when the infant cosmos underwent a major environmental change known as re-ionisation.  In the aftermath of the “Big Bang”, when cooling neutral hydrogen gas permeated all space, the Universe was re-energised by the ignition of the first stars.

These hot giants would have burnt brilliant but brief lives, producing the very first heavy elements.  At the same time, they would also have “fried” the neutral gas around them, ripping electrons off the hydrogen atoms and leaving protons.

The passage of the CMB through this maze of electrons and protons would have resulted in it picking up a subtle polarisation.


CMB Polarisation

The Planck Science Collaboration team has now analysed this polarisation in fine detail and determined it to have been generated at 560 million years after the “Big Bang”
During the 2000s, the American satellite WMAP made the previous best estimate for the peak of re-ionisation at 420 million years.  The problem with that number was that it sat at odds with Hubble Space Telescope observations of the early Universe.  Hubble could not find stars and galaxies in sufficient numbers to deliver the scale of environmental change at the time when WMAP suggested it was occurring.
Two maps of the polarization of the Cosmic Microwave Background in E-mode and B-mode.
Maps of the Polarization of the CMB. The colours show the strength of the polarization signals in one patch of the sky.  Source: Chicago University

Effectively, the two groups of astronomers were basically working on two different sides of the problem.  The Planck collaboration approached it from the “Big Bang” side, whereas those astronomers who work on galaxies came at it from the ‘now side’.

The gap in understanding had prompted scientists to invoke complicated scenarios to initiate re-ionisation, including the possibility that there might have been an even earlier population of giant stars or energetic black holes.  Such solutions are no longer needed.

No-one knows the exact timing of the very first individual stars. All Planck’s new timing does is tell us when large numbers of these stars had gathered into galaxies of sufficient strength to alter the cosmic environment.

By definition, this puts the ignition of the “founding stars” well before 560 million years after the Big Bang.  Quite how far back in time is uncertain.  Perhaps, it was as early as 200 million years.  It will be the job of the next generation of observatories like Hubble’s successor – the James Webb Space Telescope – to try to find the answer.

The new Planck result is contained in a host of recent papers posted in the Planck Legacy Archive on the ESA website.  These papers accompany the latest data release from the Planck satellite that can now be used by the wider scientific community.

Two years ago, the data dump largely concerned interpretations of the CMB based on its temperature profile.  Today, the CMB’s polarisation features takes centre-stage.

A diagram explaining the inflation and expansion of the Universe, with the associated timeline.
Cosmic inflation is the exponential expansion of space in the early Universe

Although, the Planck satellite did not find direct evidence in the CMB’s polarisation for inflation – the super-rapid expansion of space that is thought to have occurred a few fractions of a second after the initial event of the “Big Bang”, all the data – temperature and polarisation information – is consistent with that theory, and the precision measurements mean new, tighter constraints have been put on the likely scale of the inflation signal, which other experiments continue to chase.

From that point on, it just looks like the simplest models for inflation are now ruled out…