VLT Observations Confirm that the Universe Was Hotter in the Past
UVES Measures the Cosmic Temperature 12 Billion Years Ago
20 December 2000
A fundamental prediction of the Big Bang theory has finally been verified. For the first time, an actual measurement has been made of the temperature of the cosmic microwave background radiation, at a time when the Universe was only about 2.5 billion years old. This fundamental and very difficult observation was achieved by a team of astronomers from India, France and ESO . They obtained a detailed spectrum of a quasar in the distant Universe, using the UV-Visual Echelle Spectrograph (UVES) instrument at the ESO 8.2-m VLT KUEYEN telescope at the Paranal Observatory.
If the Universe was indeed formed in a Big Bang, as most astrophysicists believe, the glow of this primeval fireball should have been warmer in the past. This is exactly what is found by the new measurements.
The analysis of the VLT spectrum of the distant quasar not only gives the definitive proof of the presence of the relict radiation in the early Universe, it also shows that it was indeed significantly warmer than it is today, as predicted by the theory.
The Cosmic Microwave Background Radiation (CMBR)
One of the fundamental predictions of the Hot Big Bang theory for the creation of the Universe is the existence of the Cosmic Microwave Background Radiation (CMBR).
This relict radiation of the primeval fireball was discovered in 1964 by means of radio observations by American physicists Arno A. Penzias and Robert W. Wilson, who were rewarded with the Nobel Prize in 1978. Precision measurements by the COBE satellite later showed that this ancient radiation fills the Universe, with a present-day temperature of slightly less than 3 degrees above the absolute zero (2.7 K [Kelvin], or -270.4 °C).
This radiation comes from all directions and is extremely uniform. However, slight temperature variations in different directions have been measured, most recently by means of detailed observations from a balloon above Antarctica (the Boomerang experiment).
Since the universe is expanding, it must have been denser in the past. A particular prediction of the Big Bang theory is also that the temperature of the CMBR must have been higher at earlier times. However, although quite a few attempts have been made, no clear observational confirmation of this has been possible so far. In fact, the best observations until now have only been able to establish upper limits to the cosmic temperature at earlier epochs.
But proof is now available from new observations carried out with the Ultra-violet and Visual Echelle Spectrograph (UVES) at the 8.2-m VLT KUEYEN telescope on Paranal.
Very demanding observations
The further we look out into the Universe, the further we look back in time.
It was actually suggested more than 30 years ago that the predicted increase of temperature with distance (redshift) could be tested by observing specific absorption lines in the spectra of distant quasars.
The idea is simply that at earlier epochs, the CMBR was hot enough to excite certain atomic levels, and thus to give rise to particular absorption lines in the spectrum of a celestial object.
Some faint absorption lines of neutral carbon atoms were found to be especially promising, in the sense that they were predicted to be very sensitive to the surrounding temperature. However, previous generations of (smaller) astronomical telescopes were unable to achieve spectra of sufficient quality of these faint absorption lines in faint and remote objects in the distant (i.e., early) Universe.
The need to isolate the CMBR effects
The advent of 8-m class telescopes has now changed this situation. A few years ago, the 10-m Keck telescope (Mauna Kea, Hawaii, USA) obtained a spectrum of a quasar that was sufficiently detailed to determine an upper limit to the temperature of the CMBR at the corresponding epoch, about 3.4 billion years after the Big Bang.
However, a major difficulty of such observations is the necessity to exclude other sources of excitation (heating). It is well known that some other physical processes may also affect the observed absorption lines, such as collisions between the atoms and heating by the ultraviolet light emitted by young and hot stars.
The main problem is therefore to disentangle the various effects in order to "isolate" that of the CMBR. This can only be achieved by means of exceptionally "clean" and detailed spectra of these faint objects, an exceedingly demanding task.
For that reason all previous measurements have only led to upper limits on the CMBR temperature.
A quasar to the rescue
The new VLT spectrum of the quasar PKS 1232+0815 provides the long hoped-for break-through in this important area of cosmological research.
On its way to us, the light from this distant object is absorbed by intervening material, among other by a gaseous cloud in a galaxy at high redshift (z = 2.34). This distance corresponds to a cosmic time when the Universe was less than one fifth of its present age.
In addition to the CMBR-sensitive carbon lines, the resulting, unique spectrum shows an extraordinary wealth of other absorption lines, revealing the presence of several elements in various states of excitation. There are, in particular, a large number lines of molecular hydrogen. The multitude of information derived from these lines was the key to deducing the temperature of the CMBR impinging on the galaxy.
A subsequent, detailed analysis allowed the determination of the physical conditions in the cloud - the presence of molecular hydrogen lines was crucial for this to succeed. It clearly showed that the excitation process of atomic collisions cannot be solely responsible for the shape and strength of the observed absorption lines. An additional source of excitation must thus be present and this can only be the heating by the CMBR.
The primary outcome is therefore the first firm evidence that the relict radiation was also present in the distant past.
Moreover, it was possible to place constraints on the effect of other possible excitation processes. This made it possible for the astronomers to derive the temperature T of the CMBR at this large distance and early cosmic epoch and to place a very firm lower limit on this temperature. The final result is that T is hotter than 6 K and cooler than 14 K; this is in full agreement with the Big Bang prediction of T = 9 K.
This is thus the first real proof that the CMBR was indeed warmer in the past.
 The team consists of Raghunathan Srianand (Inter University Center for Astronomy and Astrophysics [IUCAA], Pune, India), Patrick Petitjean (Institut d'Astrophysique de Paris and Observatoire de Paris-Meudon, France) and Cedric Ledoux (European Southern Observatory, Garching, Germany).
The research described in this Press Release is reported in a research article ("Determination of the microwave background temperature at a redshift of 2.33771"), that appears in the international research jounal Nature on Thursday, December 21, 2000. The full article is also available on the web at astro-ph/0012222.
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