1. Introduction

More than 3 decades ago Fred Hoyle and the present writer began arguing the case for the widespread occurrence of microbial life in the Universe (Hoyle and Wickramasinghe, 1977, 1981, 1982, 2000). The interstellar dust grains as well as a vast array of molecules present in space were shown to represent components that may have derived from biology. If biology is not confined to Earth, and microorganisms can infect a vast multitude of habitable domains in the galaxy, such an assertion has a prima facie plausibility at least. It cannot be denied that cosmic life is a scientific hypothesis worthy of investigation and empirical test. A first test would be to ascertain the nature of the material present in interstellar dust clouds such as shown in Plate 1, which are sites where new stars form, and where molecules and dust particles are known to exist in vast quantity. It has been known for a several decades that the cosmic dust particles are of exactly the right sizes to be identified with microorganisms.

2. Interstellar Extinction

Freeze-dried bacterial cells in various stages of degradation distributed throughout the galaxy would give rise to predictable spectroscopic features. The points in Fig. 1 show the interstellar extinction curve in the form it was known since the mid-1970's, and the solid curve represents the behaviour of a biological grain model. The astronomical data, except for the ultraviolet "hump" centred at 2175A, is explained by the scattering behaviour of desiccated bacteria together with a 10% mass contribution from scattering by viral or nanobacteria-sized grains. The hump in the extinction curve at 2175A was initially attributed to small spherical graphite particles (Wickramasinghe, 1967), but later, more realistically identified as arising from aromatic molecules derived from biology (Hoyle and Wickramasinghe 1991). The entire extinction/absorption profile in Fig. 1 demands nearly 30% of the interstellar carbon to be tied up in the form of scattering and absorbing dust particles.

3. Infrared Signatures (UIB's)

The first infrared absorption spectrum indicating cosmic biology was obtained for the Galactic Centre source GCIRS7 by D.T. Wickramasinghe and D.A. Allen (Wickramasinghe and Allen, 1980; Allen and Wickramasinghe, 1981), and interpreted as biological material by Hoyle and Wickramasinghe (1979, 1980, 1991). For reasons of historical interest this comparison is reproduced in Fig. 2. The solid curves show the close correspondence over the waveband 2.8-12 µm (Willner et al, 1979), and the inset shows the detailed correspondence with the astronomical data over the 2.9-3.9 µm waveband (Allen and Wickramasinghe, 1981). From the measured value of the absorption coefficient of desiccated bacteria at 3.4 micrometres of ~ 500 cm²g-1, and the astronomical data in Fig. 2 we can readily infer that about 25-30% of all the interstellar carbon in the line of sight of GC-IRS7 must be in the form of bacterial-type grains. Spectroscopically at least the interstellar grains are indistinguishable from bacteria.

With improved techniques of observation the infrared region of astronomical sources has been more thoroughly investigated in recent years, for instance by the ESA infrared space observatory and the Spitzer Space telescope. Biological signatures have continued assert themselves in a wide range of astronomical spectra. The inference of an all pervasive cosmic life implied in the correspondences shown in Figs 1 and 2 continue to become stronger with the arrival of new data (Smith et al, 2007). Attempts to explain much of this data in terms of abiotic processes producing organic molecules, polymers and PAH's remain contrived and unproven.

A set of unidentified infrared emission bands (UIBs) between 3.3 and 22 μm is found in almost every dusty region of the galaxy as well as in the spectra of external galaxies. Recent observations of such UIBs for a large number of galactic and extragalactic sources have been obtained using the Spitzer Space Telescope (Smith et al., 2007). Fig. 3 shows the spectra of a young planetary nebula and a protoplanetary nebula showing characteristic UIB's (Kwok, 2009). The comparison of the UIB wavelengths with naturally occurring biological systems replete with PAH's is shown in Table 1.

4. Origin of PAH's

The origin of PAH's such as could result in spectra such as Fig. 3 is still a subject of intense debate and controversy. Whilst PAHs (Polyaromatic Hydrocarbons), presumed to form inorganically, are the currently favoured model for the UIBs, satisfactory agreement with all available astronomical data has not been possible (Hoyle and Wickramasinghe, 1991). As seen in Fig 4 the positions of the absorption bands corresponding to any given PAH (eg coronene, cirocbiphenyl, dicoronene and ovalene) are known to correspond to some of the UIB's, but the precise wavelengths are sensitive to excitation conditions and ionization states.

5. Diffuse Interstellar Absorption Bands (DIB's)

The diffuse interstellar absorption bands in optical stellar spectra, particularly the well known 4430A feature, also have possible explanations on the basis of molecules such as porphyrins (Hoyle and Wickramasinghe, 1979; Johnson, 1972). Since their discovery in 1936 the number of these bands, with widths in the range 40A – 2A, spanning the waveband from 4400A to 7000A, has increased to well over two dozen. The bands are too wide to be explained in terms of electronic transitions in atoms, ions or small molecules, and to this day their identification and origin remain obscure. The widths and central wavelength placements of the main DIB's is shown in Fig 5 (Herbig, 1995).

With a shift of interest in recent times from optical astronomy to observations in the infrared and ultraviolet, the DIB's appear to have been somewhat neglected. However, from the considerable dataset already available, the role of PAH's (or biological pigment including a specific set of PAH's) appears to be likely (Destree and Snow, 2007; Herbig, 1995; Hoyle and Wickramasinghe, 1991). In the view of the writer the most promising class of models is one that was suggested by F.M. Johnson nearly four decades ago (Johnson, 1971, 1972). Even a cursory glance at the spectral data for chlorophylls, metalloporphyrins and related pigments shows that the strongest of the DIB's could arise from such systems – particularly the bands at 4428A, 6175A and 6614A (Hoyle and Wickramasinghe, 1991). Johnson (1972) has argued these bands could arise from electronic transitions involving the molecule MgC₄₆H30N₆ . At a time when no molecule more complex than formaldehyde was known to be present in interstellar space, it is not surprising that Johnson's suggestion was ridiculed and immediately dismissed.

6. Extended Red Emission (ERE)

Another feature of astronomical spectra that points to cosmic biology is the so-called extended red emission (ERE). A fluorescence phenomenon over the red waveband 5000-7500A was first observed in dusty regions of the galaxy by Witt and Schild (1985, 1988) and this data has now been considerably expanded. Extended red emission has been observed in planetary nebulae (Furton and Witt, 1992), HII regions, dark nebulae (Matilla, 1979; Sivan and Perrin, 1993) and high latitude cirrus clouds (Szomoru and Guhathakurta, 1998) in the Galaxy as well as in extragalactic systems (Perrin, Darbon and Sivan, 1995; Darbon, Perrin and Sivan, 1998). This phenomenon has a self-consistent explanation on the basis of fluorescence behaviour of biological chromophores (pigments), e.g. chloroplasts and phytochrome (Fig. 6A) (Hoyle and Wickramasinghe, 1991; Wickramasinghe et al., 2002). Competing models based on emission by compact synthetic PAH systems are not as satisfactory, as is evident in the example shown in Fig. 6C. This particular example is for the PAH model involving cationic dimmers of pyrene-phenalenyl (Min Rhee et al 2007) but most other compact PAH models – eg hexa-peri-bensocononene – suffer the same problems. Not only have synthetic PAH's to be arbitrarily chosen but their ionization states and excitation conditions have also to be arbitrarily prescribed. Even with such provision, the width and central wavelength of the fluorescent emission leave much to be desired. Thus biology appears to afford the simplest explanation of this phenomenon, and we appear to have the entire cosmos glowing like fireflies seemingly declaring its biological connection.

7. Comet Spectra

Since the 1986 perihelion passage of comet P/Halley, spectroscopy of comets and cometary dust has also revealed an astounding consistency with cosmic biology. As with spectra of interstellar dust an ambiguity in the assignment of a broad 9.7 µm and 18 µm infrared features to silicates has clouded the issue from the outset. The C-O-C, C-C bonds in biopolymers have nearly coincident resonant frequencies with O-Si-O vibrations, but with the cosmic Silicon abundances down on Carbon by a factor 30 and with consistently better fits to astronomical spectra demonstrated for biomaterial, there was hardly any competition between the two explanations (Hoyle and Wickramasinghe, 1991).

Of course silicates must be present in interstellar space as well as in comets, but their abundance is much lower than that of biological material. Fig. 7 shows the spectra of comet Hale Bopp (jagged curve) and of comet Halley (points in inset) over the wavebands 8-45 microns and 3-4 microns respectively (Wickramasinghe and Allen, 1986; Hoyle and Wickramasinghe, 1991, Crovisier et al, 1997). The dashed curve (main graph) is for a bioculture of diatoms and bacteria with a 10% mass contribution from silicates. The inset shows a comparison of the 3-4 micron data for comet Halley with bacterial grain models (curves). Spectroscopic data showing comets to contain organic dust that is indistinguishable from bacteria support the theory that comets are the amplifiers and distributers of cosmic life – theory of cometary panspermia (Hoyle and Wickramasinghe, 2000; Wickramasinghe, 2010).

8. Extragalactic Biology

How far does the spectroscopic evidence of biology extend? We have noted that the galaxy is evidently full of biotic-type material and that some 25-30% of the carbon in the ISM must be in this form. Extragalactic sources also have the same signatures of living material. Fig. 8 shows the extinction curves of the Large and Small Magellanic clouds at distances of 180 and 210 kpc respectively showing the 2175A aromatic absorption band to varying degrees, as well as a background of scattering consistent with desiccated bacteria.

Moving further out to some 21Mpc there is a particularly interesting case of the Antennae galaxies (Fig. 9) where the infrared emission spectrum matches the laboratory spectrum of anthracite (Guillois et al., 1999). Anthracite being a product of biological (bacterial) degradation is again indicative of cosmic biology, in this case at great distances from the Milky Way.

The average 8-12 micron excess absorption spectra of dust is shown in Fig 11 compared with the prediction from a clay-biomaterial mixture (J.T. Wickramasinghe, et al, 2009). The spectrum of the biomaterial is taken from measurements of a mixed culture of desiccated microorganisms (bacteria and diatoms) (Hoyle and Wickramasinghe, 1991). The mass fraction of silicates in the mixture is less than 10% . Figure 12 shows the average spectrum of 13 starburst galaxies showing the placements of UIB's. The existence of these bands, which can be assumed to result from the degradation of biomaterial, is an indication that cosmic biology extends at least to a distance of ~ 100Mpc

The most dramatic discovery of recent times that relates to astronomical aromatic molecules is a conspicuous 2175A absorption band in the lens galaxy of the gravitational lens SBS0909+532 which has a redshift of z = 0.83 (Motta et al., 2002), and a similar feature in a CO cloud associated with a quasar SDSS J160457.50+220300.5 at redshift z=1.64 (Naterdaeme et al, 2009). The two spectra are reproduced in Fig 13.

In Fig 14 we show the comparison of the normalized absorption profiles of these two sources compared with the properties of biological aromatic molecules (Wickramasinghe et al., 1989).

The close match between the data points for the two sources and the theoretical curve shows that biological aromatics are a viable explanation of an absorption feature of material located at a redshifts of z = 0.83 and z=1.64. These redshifts represent epochs when the universe was respectively one half and one quarter of its present radius. The implication is that the origin of life is an event that takes place on a cosmological scale, perhaps only once, or at most a few times in the history of the Universe. The arguments for terrestrial life having an evolutionary history in excess of 10 billion years (Joseph, 2000, 2010; Joseph and Schild 2010a,b), and for an unique origin of life in the Gibson-Schild version of Big-Bang cosmology are in accord with these findings (Gibson, Schild and Wickramasinghe, 2010; Wickramasinghe, Wallis, Gibson and Schild, 2010).

Conclusions

The most persistent spectral spectroscopic features in astronomical sources that are consistent with biology are summarized as follows:

The logical connections of cosmic biology to different astronomical data sets is summarized in Fig. 15. The identification of PAH's and organic molecules in (1)-(6) with biological particles and their degradation products is expected on the basis of panspermia theory. With approximately 25-30% of the carbon in interstellar space being tied up in the form of PAH's and complex organic polymers, attempts to explain this data avoiding biology must verge on being perverse. The Universe is asserting our cosmic ancestry unequivocally through every spectroscopic examination that we have devised (Wickramasinghe, 2010). We can no longer afford to ignore these facts as we have been accustomed to do for decades.