Những bài cùng tác giả

Figure 1: Image of the spiral galaxy NGC 613 in the Southern Constellation Sculptor taken with the VLT telescope of ESO (Credit: M. Neese, P. Barthel, H. Heyer, and H. Boffin).

Thousands of bright stars in the Milky Way can be seen with the naked eye. Billions of other stars and galaxies in the Universe remain invisible without the use of large telescopes. Astronomical objects do not emit only light but also radiation over the whole electromagnetic spectrum. Their investigation requires the observations at as many wavelengths as possible, from gamma, X-rays, ultraviolet and visible radiation to infrared and radio waves. The characteristics of the cosmic emission depend on the physical conditions, ẹg., the temperature, the density of matter and the magnetic field. Because most of the electromagnetic radiation coming from outer space is either absorbed or reflected by Earth's atmosphere, only two narrow spectral windows, namely the visible-near-mid infrared and radio wavebands, are almost transparent to cosmic radiation. Outside these windows, the observations must be performed with space telescopes.

The Universe is an immense laboratory, in which various physical conditions exist; it allows all kinds of phenomena to take place, even those which cannot be produced in laboratories on Earth. Astrophysics involves many fields, from mathematics, physics, chemistry to biology. One of the main branches of astrophysics is cosmology, which investigates the origin and the evolution of the Universe, as well as the nature of matter and energy. Other topics concern the study of the physical processes at work in galaxies, stars, the interstellar medium and dense objects such as black holes. Searches for planets outside the solar system and for possible life in the Universe have also been undertaken. Large telescopes with high sensitivity detectors and high resolution spectrometers that operate at wavelengths covering the whole electromagnetic spectrum are required for these purposes.

Cosmology

Hubble discovered that the Universe is in expansion as shown by the motion of galaxies receding from one another. Their radiation is shifted towards long wavelengths (ređshifted, ịe., shifted towards the red end of the electromagnetic spectrum), according to the Doppler law. There are two main competing cosmological concepts. In the steady-state model, the creation rate of matter in the Universe balances the decrease of its density due to the expansion, such that the overall Universe remains unchanged. On the other hand, the Big Bang model advocates that the Universe was spontaneously created about 14 billion years ago from a state of infinitely high density and temperature. Since then, the Universe has been continuously expanding and cooling down.

The Big Bang theory, though perfectible, resists better against the confrontation with observation and is, therefore, adopted by the great majority of astrophysicists. At the beginning, the Universe was extremely hot and matter was ionized. In this primordial plasma, photons could not propagate without interacting with free electrons. The Universe was therefore opaque during the first 400,000 years, until the temperature decreased to ~ 4,000 Kelvin. At this temperature, electrons started recombining with ions to produce neutral atoms, allowing the Universe to become transparent.

Gravity is a universal force in nature, holding galaxies together in clusters. The geometry and the evolution of the Universe are governed by Einstein's equations of general relativity involving matter and energy. Since these equations cannot be completely solved generally, simplified models were proposed. The most famous one is Friedmann's model which assumes that the Universe is homogeneous and isotropic on a large scale. The Universe starts as a singular point identified to the Big Bang, then expands and can even collapse under the gravitational attractive force of matter. This model turns out to be consistent with the discovery of the Cosmic Microwave Background (CMB) radiation, which is ubiquitous over the sky. This fossil radiation is the remnant of the Big Bang detected by radio astronomers in 1965. Later the observations with the satellites COBE, WMAP and balloons revealed that the CMB is actually not homogeneous. The inhomogeneities in the primordial Universe are the seeds of clusters of galaxies that we are observing nowadays.

The hot Big Bang model has been improved to explain in detail the large scale structure of the Universe. Instead of expanding uniformly, the primordial Universe experienced, shortly after the Big Bang, a sudden phase of acceleration during which the size of the Universe increases exponentially. The existence of this event, called "inflation", was proposed to explain the large scale uniformity of the CMB and the flatness of the Universe as observed. The lumpy small scale structure of the Universe is associated to the quantum fluctuations which were amplified during the inflation phase and eventually collapsed to form galaxies.

The string theory advocates that the basic element of matter exists in the form of tiny strings, instead of point like elementary particles. The dimensions of cosmic strings are negligibly small, compared to the characteristic scales involved in standard experiments. The string theory requires that space-time has at least ten dimensions, instead of four as usual. The extra dimensions are highly curved and hidden. Strings are small but have very high tension. The properties of different well-known elementary particles are produced by various modes of vibration of strings. The string theory would lead to a quantum theory of gravitation, making it possible to investigate the Universe at its early stage close to the Big Bang, when the density and the curvature of the Universe were infinitely large. The string theory may become a "theory of everything", unifying all fundamental forces, ịe., the weak and strong nuclear, electromagnetic and gravitational forces. However, the confirmation of the existence of strings by experiments is still far beyond the scope of existing particle accelerators.

Bright supernovae of a particular type (Type Ia), exploding in remote galaxies, have been used as indicators of distance. They have the same intrinsic luminosity, and their apparent brightness depends only on their distance. It turns out that supernovae appear fainter than expected from the measurements of the redshift of their spectrum. These results suggest that the observed supernovae are actually farther than predicted and the expansion of the Universe is accelerating. Theoretically, the acceleration is due to a parameter, the so called "cosmological constant lambda", corresponding to a repulsive force initially introduced by Einstein in his equation to balance the gravitational attractive force in order to obtain a solution for a static Universe. Physically, it is associated with a kind of energy, the dark energy, which accelerates the expansion of the Universe. This energy is believed to be related to the vacuum energy familiar to high energy particle physicists. Moreover, the observations of the CMB and the supernovae indicate that the Universe consists of 70% of dark energy and 30% of matter, mostly invisible dark matter. These data are consistent with a flat Universe dominated by dark energy and to some extent by dark matter. However, the nature of the dark components still remains unknown. "Baryonic" matter that is the ordinary matter, constituent of stars and interstellar matter, including atoms and molecules inside the cells of living beings on Earth, amounts to only 4%.

The interstellar medium

The interstellar medium, which contains essentially gas and dust, is also a subject of investigation of major importance. Its average density and temperature are a few ten hydrogen atoms per cubic centimetre and a few ten Kelvin, respectively. These values are very low in comparison with those we know in the Earth environment. The density of the interstellar medium is about ten billion billion times lower than the density of the air we breathe on Earth, where the mean temperature is above 273 Kelvin (0 degree Celsius). Light atoms mainly hydrogen, deuterium and helium were formed during the very first minutes after the Big Bang. The first heavier atoms as well as molecules were manufactured hundreds million years later, by the first generation of stars, and subsequently injected into the interstellar medium during the late stage of stellar evolution, in particular through supernova explosions. Search for interstellar molecules was made extensively at radio wavelengths, since radio lines are easily excited by collisions between molecules and hydrogen as well as radiation. Since four decades, more than a hundred of molecules, among them many organic molecules, have been detected in the Milky Way. Of particular interest is the detection of acids and amines such as formic acid (HCOOH) and methylamine (CH3NH2), which are the fragments of the glycine molecule (NH2CH2COOH). In the glycine molecule, the side chain group (attached to a carbon atom) is a single hydrogen atom, instead of a complex group of atoms. Glycine is the simplest member of the family of 20 standard amino acids found in proteins.

Since three decades, searches for glycine have been made by several groups of radio astronomers in the direction of star forming regions, such as the Orion Nebula and the centre of our Galaxy. These targets are the sites where many complex molecules were detected and therefore appear to be promising for the search for glycine. So far, no evidence has been found for the existence of glycine in space. It is likely that the spectral lines of glycine are too weak and mixed with the background made of other weak molecular lines, causing a spectral confusion. Glycine can be considered as a biomarker, a kind of signature of life. Its detection in the interstellar space would have a great impact not only on astrochemistry, but also on the issue of the origin of life on Earth and possibly elsewhere.

Search for extra-solar planets and tracers of biological activities

The question we may ask is whether or not life exists somewhere else in our Galaxy, which contains myriads of stellar systems similar to the solar system with planets orbiting around. The development of the type of life like that existing on Earth is a very long process. The solar system was born 4.6 billion years ago, but the first human beings only appeared about 3 million years ago. Life can only emerge on a planet which has appropriate physical and chemical conditions. The planet must lie neither too close nor too far from the central star, in a region of the stellar system called the "habitable zone", such that the physical conditions are favourable to harbour life. For example, in the solar system, the frontiers of the habitable zone are between 120 million and 250 million kilometres from the Sun (0.8 and 1.6 times the radius of the Earth orbit). The Earth is in the middle of this zone while Venus and Mars lie at the edges. Furthermore, the planet should have oxygen in its atmosphere and liquid water on its surface, the two elements essential for life.

Robotic rovers were launched to explore the surface of nearby planets in the solar system, in particular Mars. They found evidence for the existence of water, which is all evaporated nowadays. The Huygens probe was released from the Cassini spacecraft to probe the atmosphere and the surface of Titan, the biggest satellite of Saturn. Space observations revealed the presence of hydrocarbons in Titan's atmosphere and dark structures on its surface believed to be lakes filled with liquid methane. This organic species is in the gas phase on Earth, but the very low temperature on Titan, about -180o Celsius, maintains methane in the liquid phase. These results indicate that extra-terrestrial lakes may exist on Titan, but do not necessarily contain water. The environment of Titan does not seem to favour the kind of life similar to that existing on Earth.

Planets do not emit light by themselves since they are not as hot as stars to trigger thermonuclear reactions. They simply reflect radiation coming from the central star and some of them may harbour life. Therefore, the detection of planets outside the solar system (extra-solar planets or exoplanets) is a prerequisite to the search for life in space. The central star is billions times brighter than the companion planets. The contrast between the brightness of the star and the planets is so important that it is quite difficult to detect the latter. The observations can be made in the mid infrared bands (~ 10 micrometre) where the contrast is thousands times lower than in the visible. Another difficulty is the ability to distinguish the planets from the parent stars whose angular separation is very small. For example, if the Earth is observed from a distance of 30 light-years, it would be separated from the Sun by only 0.1 arcsec. Such angular resolution requires the use of a huge telescope of 20 m in diameter.

In order to avoid these difficulties, astronomers have used an indirect method which consists in detecting the perturbation of the motion of the parent star caused by the unseen planets. They measure the periodic change in radial velocity (velocity along the line of sight) of the star, as a result of the changing direction of the gravitational pull from the orbiting planets. This method favours actually the detection of big planets whose motion perturbs more easily the central star. So far more than 200 planets have been discovered by this technique, most of them are much bigger than the giant planet Jupiter, which is 320 times more massive than the Earth. Recently, astronomers have used a high resolution spectrometer to detect velocity changes of the order of a few meters/second. They discovered an extra-solar planet as massive as only 5 times the Earth. Judging from its relatively low mass, this object, which belongs to the stellar system GI 581, is probably a rocky planet of the type of the Earth. Furthermore, this "Super Earth" lies in the habitable zone of the stellar system and may well harbour life. The detection of Earth-size planets requires spectrometers capable of detecting a velocity variation with an accuracy of ~ 0.1 m/s. This performance cannot be achieved with radial velocity techniques.

There exists another method to detect extra-solar planets. It consists in observing the very weak dimming of the starlight when the orbiting planet passes in transit in front of the central star. A satellite dedicated to the detection of planets by the transit method was launched by ESA (European Space Agency) in 2006. Astronomers expect to observe hundređthousands of stars and hope to detect rocky planets as small as the Earth.

The most direct way to detect extra-solar planets in a stellar system is to extinguish the light from the bright central star and let the faint companion planets appear. This is the objective of the ambitious ESA space project Darwin which uses the interferometry technique to remove the starlight. The Darwin system consists of six telescopes placed in a hexagonal configuration, 1.5 million km away from the Earth. The whole system revolves around the Sun at the same velocity as the Earth. The telescopes operate in correlation as an interferometer array, producing bright and dark fringes. The array can be tuned such that the light from the direction of the star interferes destructively (that is a dark fringe coincides with the position of the star), while the light from the direction of the planet interferes constructively (a bright fringe on top of the planet). This sophisticated technique, though routinely used in radio astronomy on Earth, is difficult to operate in space. It requires the position of each telescope to be maintained as stable as possible. Darwin is also intended to analyse the atmosphere of the detected Earth-like planets and search for the elements like oxygen, ozone, carbon dioxide, methane and water. These molecules are the markers of biological activities as we know on Earth. Darwin will be a very promising project to discover the planets where the physical conditions may lead to the emergence of extra-terrestrial life.

Prospects

Cosmic radiation received on the telescope and radio telescope is extremely weak. Remote galaxies are as faint as the flame of a candle placed on the Moon and observed from the Earth. As to their radio emission, it can be billions times weaker than the television signal. Interferometry technique extensively used nowadays combines an array of telescopes operating in correlation and increases dramatically the spatial resolution. Molecular radio emission from star forming regions has been found to be greatly amplified by maser effect. These powerful cosmic masers are made up of very small hot spots. Interferometry is particularly appropriate to their investigation. The radio interferometer ALMA, one of the largest international projects in radio astronomy of the next decade, is composed of sixty four 12-m antennae operating at millimeter wavelengths. It is being installed on a plateau at 5,000 m elevation in northern Chili. The antennae can be placed on hundreds of stations spread over 18 km. This new generation of giant radio interferometers is designed to study faint objects in details. The mechanism of formation of stars and planets, and the radiation of gas and dust from galaxies formed at the very early epochs close to the Big Bang can be studied with this instrument.

Interferometry is also used in the optical domain. The Very Large Telescope (VLT) located in northern Chili in a desert at 2,600 m elevation consists of four 8-m mirrors and a few smaller telescopes. These instruments can work in the interferometric mode, allowing astronomers to detect faint fine structures in celestial objects, equivalent to an astronaut walking on the Moon. The study of the origin and the evolution of the Universe, and the search for earth-like planets are among possible scientific targets.

Thanks to large instruments installed on Earth and launched in space, equipped with state-of-the-art detectors made of semiconductor and superconductor materials, together with theoretical developments, astronomers will probe more deeply the Universe (Fig. 1) and expect to witness the discovery of unexpected and fascinating phenomena.

References

1. Françoise Combes, Nguyen-Quang-Rieu and Georges Wlodarczak: 1996, Search for interstellar glycine (Astronomy & Astrophysics, Vol. 308, p. 618).

2. Fred Hoyle, Geoffrey Burbidge and Jayant V. Narlikar: 2000, A Different Approach to Cosmology from a Static Universe through the Big Bang towards Reality (ed. Cambridge University Press).

3. Patrick Peter and Jean-Philippe Uzan: 2005, Cosmologie primordiale (ed. Belin, Paris).

4. S. Udry, X. Bonfils et al. 2007, The HARPS search for southern extra-solar planets. Super-Earth (5 & 8 Earth masses) in a 3-planet system (Astronomy & Astrophysics, Vol. 469, Nọ3, p. L43).

Đã đăng trên Viplok ngày 7/7/2007