Cosmology. The Origin and Evolution of Cosmic Structure - Coles P., Lucchin F

458 A Forward Look

As a final comment on SKA science, it is worth mentioning the enormous advantage it has for gravitational lensing studies. Not only does it have a much larger field than comparable optical/IR facilities but it also has a very well-defined pointspread function which will enable higher signal-to-noise measurement of individual galaxy ellipticities.

21.10 Gravitational Waves

We have touched briefly on gravitational waves a few times during the course of this book, largely in connection with their possible production during inflation and role in the production of anisotropies in the cosmic microwave background. Most physicists think that gravitational radiation must exist, although they are yet to be detected directly. One of the important results to emerge from Maxwell’s theory of electromagnetism was that it was possible to obtain solutions to Maxwell’s equations that describe the propagation of an electromagnetic wave through a vacuum. Analogous solutions can be obtained in Einstein’s theory, and these represent what are known as gravitational waves or, sometimes, gravitational radiation. The properties of, and searches for, gravitational radiation constitute a rich field all of their own so we cannot give a complete picture here (see, for example, Thorne 1987). What we will do is give a quick summary of their properties and focus on some of the possibilities for gravitational wave cosmology, if and when such radiation is directly detected.

Gravitational waves represent distortions in the metric of space–time in much the same way that fluctuations in the density of matter induce distortions of the metric in perturbation theory. The metric fluctuations induced by density fluctuations are usually called scalar perturbations, whereas those corresponding to gravitational waves are generally described as tensor perturbations. The reason for this di erent nomenclature is that gravitational waves do not result in a local expansion or contraction of the space–time. Scalar perturbations can do this because they are longitudinal waves: the compression and rarefaction in di erent parts of the wave correspond to slight changes in the metric such that some bits of space–time become bigger and some smaller. Gravitational waves instead represent a distortion of the geometry that does not change the volume. In technical terms, they are transverse-traceless density fluctuations. (Vector perturbations correspond to vortical motions which are transverse, but not trace free.) Gravitational waves are similar to the shear waves one finds in elastic media: they involve a twisting distortion of space–time rather than the compression seen in longitudinal scalar waves.

Gravitational waves are produced by accelerating masses and in situations of rapidly changing tidal fields. The more violent the accelerations involved the higher the amplitude of the gravitational waves. Because Einstein’s theory of general relativity is nonlinear, however, the waves become very complicated when the amplitude gets large: the wave begins to feel the gravitational e ect produced by its own energy. These waves travel at the speed of light, just as electromagnetic radiation does. The problem with detecting gravitational waves, how-

ever, is that gravity is very weak. Even extremely violent events like a supernova explosion produce only a very slight signal. Gravitational-wave detectors have been built that attempt to look, for example, for changes in the length of large metal blocks when a wave passes through. The expected signal is much smaller than thermal fluctuations or background noise, however, so such experiments are extremely di cult. In fact, the typical fractional change in length associated with gravitational waves is less than 10−21. Despite claims by Weber in the 1960s that he had detected signals that could be identified with gravitational radiation, no such waves have yet been unambiguously observed. The next generation of gravitational wave detectors such as GEO (a UK–German collaboration), Virgo (France/Italy) and LIGO (USA) should reach the desired sensitivity using interferometry rather than solid metal bars. The LIGO experiment, for example, involves an interferometer with arms 4 km in length. Moreover, plans exist to launch satellites into space that should increase the baseline to millions of km and thus increase the sensitivity to a given fractional change in length. One such proposal called LISA is pencilled in for launch by the European Space Agency sometime before 2020.

Although these experiments have not yet detected gravitational radiation, there is very strong circumstantial evidence for its existence. The period of the binary pulsar 1913 + 16 is gradually decreasing at a rate which matches to great precision relativistic calculations of the expected motion of a pair of neutron stars. In these calculations the dominant form of energy loss from the system is via gravitational radiation, so the observation of the ‘spin-up’ in this system is tantamount to an observation of the gravitational waves themselves (Taylor et al. 1979). Hulse and Taylor were awarded the Nobel Prize for studies of this system in 1993.

As we mentioned above, it is also possible that gravitational waves have already been seen directly. The temperature fluctuations seen in the cosmic microwave background radiation are usually attributed to the Sachs–Wolfe e ect produced by scalar density perturbations; see primordial density fluctuations. But if these fluctuations were generated in the inflationary Universe phase by quantum fluctuations in a scalar field, they are expected to be accompanied by gravitational waves which in some cases could contribute an observable Sachs– Wolfe e ect of their own. It could well be that at least part of the famous ripples seen by the Cosmic Background Explorer (COBE) satellite is caused by gravitational waves with wavelengths of the same order as the cosmological horizon.

It can be speculated that in a theory of quantum gravity the quantum states of the gravitational field would be identified with gravitational waves in much the same way that the quantum states of the electromagnetic field are identified with photons. The hypothetical quanta of gravitation are thus called gravitons. It has been argued that gravitational wave astronomy could push back the frontiers of the observable universe from the epoch of recombination to the Planck epoch, since gravitons are expected to decouple at the latter energy scale.

460 A Forward Look

21.11 Sociology, Politics and Economics

We hope it is apparent that there are many exciting developments on the horizon, and that cosmology can look forward to a vigorous and challenging future. But as well as the forthcoming developments in technology, the years to come will probably also lead to changes on the human side of the subject. Science, after all, is a very human kind of activity and we could not resist the temptation to speculate a little about the likely impact on how astronomy is performed.

The new technology that has driven observational astronomy at the breakneck pace it has enjoyed over the last decades has also led to changes in the way the accompanying human resources are organised. Collaborations are now very much larger than they were even 20 years ago, leading to di culties in bringing younger scientists through to prominence and assigning credit to individual contributions. This trend is likely to continue, with monolithic survey projects involving dozens if not hundreds of scientists becoming the rule rather than the exception for leading-edge research in astronomy. This is also becoming the case in theory, especially in respect of the large collaborations involved in supercomputer simulations of structure formation.

The organisation and control of access to astronomical facilities may also change dramatically, as more dedicated high-cost facilities take the place of multipurpose facilities whose time is allocated by peer-review processes of various kinds. With more and more observational programs being constructed in response to specific science goals, often strongly informed by theoretical ideas within a specific framework, the role of serendipitous discovery seems set to diminish. Altogether these factors conspire against the creative maverick and in favour of the conformist team player. Whether one thinks this is a good thing or a bad thing depends on one’s own personality.

There is also a more subtle change of emphasis, which can be seen even in the structure of this chapter. More for presentational purposes than anything else, we organised the discussion by wavelength region. This is an increasingly outdated way of thinking. Future science programmes are likely to be much more organised by science goal than by wavelength region. Traditional communities, such as radio astronomy and X-ray astronomy, will see their boundaries blurred by the growing number of scientists driven by an interest in particular objects rather than particular kinds of photon.

So much for sociology, how about politics and economics? The main point that comes to mind relates to the cost of these facilities. By any criteria, all the missions and facilities we have discussed are extremely expensive. For this reason, as much as any intrinsic transnationalism between scientists, upcoming developments are likely to be multinational in character. ALMA is a true world astronomy project, involving substantial financial investments from many countries including the ESO member countries and the USA. The SKA has an even broader distribution of likely contributors. These coalitions are brought together by the impossible strain on budgets of individual countries that would be caused if they took on projects

Conclusions 461

of such a scale on their own. But even if global collaborations are possible, there must be some limit to the amount of cash that can be assembled for scientific studies, especially in times of global recession. When will we reach that limit, and what will be able to learn before we do?

21.12 Conclusions

This has been a very superficial and biased review, but we hope it has given some insights into the way extragalactic astronomy might head over the next decade or two. We have refrained from attempting to give accurate dates, because these are so likely to be revised as to make such guesses worthless.

It is astonishing how much things have changed over the last decade and a half. In 1985 the largest redshift survey available comprised a thousand galaxies or so and fluctuations in the cosmic microwave background were not yet detected. In some sense, that was a very good time to be a theorist but it was clear then that, compared with other sciences, cosmology was extremely immature. Now, with a steadily growing empirical foundation and an exciting interplay between theory and observation, it is has come of age as a science. Its future development promises much and, rightly, it is observation that will drive it forward.

Appendix A

Physical

Constants

The usual symbol for the radiation density constant is a, but this would clash too frequently with our use of a for the cosmic scale factor in this book, so we have chosen to call it σr.

The fine-structure constant is

in SI units.

464 Appendix A

The cross-section for weak interactions in thermal equilibrium at temperature T is given by

σwk = g2 kBT 2 m2, wk ( c)2

where gwk is the weak coupling constant. These are mediated by the W± and Z0 bosons which have masses 80.6 GeV and 91.18 GeV, respectively.

Appendix B

Useful

Astronomical

Quantities

Properties of the Sun

Other astronomical quantities

Appendix C

Particle

Properties

The standard model of particle physics has three families of quarks organised in doublets (u,d), (s,c) and (b,t). These have the following properties:

Quarks are confined in hadrons, which are either mesons (q¯ pairs) or baryons (q1q2q3 triplets). Familiar examples of the baryons are the proton (uud) and the

The pions have masses around 136 MeV. Since the quarks carry a colour charge (either red, green or blue) these can be constructed to be either colour–anticolour combinations (mesons) or mixtures of three colours (baryons). Either way the resulting states are colourless.

There are also three families of leptons, organised in doublets (e, νe), (µ, νµ) and (τ, µτ ). These have the following properties: