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Origin (?) of the Universe 2 Jayan! V Narfikar Origin (?) of the Universe 2. The Expanding Universe Jayant V Narlikar The expansion of the universe was established based on observations made in the 1920's, ofthe Doppler shift oflight from galaxies. The proportionality between velocity and distance is the famous Hubble Law. Simplified mathemati­ cal models of the universe are based on the idea, which is supported by observations, that there are no preferred loca­ Jayant Narlikar, Director, tions or directions in space. Inter-Univenity Centre for Astronomy and Hubble's Law Astrophysics, works on action at a distance in physics, new theories of By the beginning of the present century, astronomers had two gravitation and new important sources of information. Photographic plates more models of the univene. efficient than the human eye supplied images of faint objects He has made strong efforts to promote teaching and while spectrographs analysed the light forming these images into research in astronomy in components of different wavelengths. Before proceeding to the univenities and also Hubble's Law let us note how these two sources of information writes extensively in helped early extragalactic astronomy. English and Manthi for a wider audience on science and other topics. The spectra of nebulae were first investigated in 1864 by William Huggins by visual inspection. Only the continuous distribution was clearly visible; the lines, if any, were too faint to be detected. The brightest of the nebulae, M31, was subsequently investigated in detail. (M31 is the catalogue number of the galaxy in the This six-part series will compilation made by Charles Messier). In 1899, J Scheiner cover: 1. Historical Back­ established the presence of absorption lines in the spectrum of ground. 2. The Expanding M31, although he was unable to measure their precise wave­ Universe. 3. The Big Bang. lengths. 4. The First Three Minutes. 5. Observational Cosmol­ The breakthrough was made in 1912 by V M Slipher at Lowell ogy and 6. Present Chal­ Observatory, using a fast camera attached to the observatory's 24- lenges in Cosmology. inch refracting telescope. When Slipher measured the wave- -6--A-pp-e-a-re-d-j-n-V-O-I.'-, -NO-.-2'-P-P-.'-2--'7-,-'9-9-6--~-~----R-E-S-O-N-A-N-C-E-I-D-ec-e-m-b-e-r -2-0-05 ~, Origin (?) of the Universe X~'<, ~ lengths of the absorption lines in M31, he found a some­ The spectra of what unexpected result. The spectral lines appeared to be nebulae were first displaced from their natural locations, their observed wave­ investigated in 1864 lengths being systematically shorter than the expected by William Huggins (signature) values. The shift was thus towards the violet by visual end of the spectrum. M31 was however the odd one in a inspection. Only the steadily growing list of nebulae studied by Slipher, for, by continuous 1925, Slipher's list of such shifts of absorption lines in the distribution was spectra of nebulae like M31 had grown to forty one, and it clearly visible; the had become clear that the general rule was of a shift in the lines, if any, were opposi te direction, towards the red end of the spectrum. too faint to be Such a redshift may be defined quantitatively as the frac­ detected. tional increase in the original wavelength. Cluster Distance in Redshifts nebula in light-years H+K 78,000,000 I I I ,~~, I I I CVirgo 1,200 km 5-1 III I f I II I 1,000,000,000 .... ""~~ III I i I II I Ursa Major 15,000 km 5-1 • I! I I ! 1.400,000,000 ._-_ .. _---- Figure 1 Redshiff-Faint­ II. I i I neu Relation: Hubble's ob­ Corona 22,000 km 9-1 Borealis servatiOMdemoMfratehow • I I 1,1 II I Ii! J las we go down the figul'tlJ 2,500,000.000 - - - - ~.- for fainter and fainter gal­ III ill I II I axla (shown on the leff), Bootes 39,000 km s-' the shiff of absorption lines I 1/ I I I I I I in thespedra Ishown on the 3,960,000,000 ..-----. -- - -. rightJ incf'f!H1$ll$. Hubble con­ I I I I I I i I I verted falntneu Info dis­ Hydra 61,000 km 5' tance of the galaxy, and Red - .hlft. 0" ..pr."ed o. Vllocltie., c: d>'/>'. Arrow. Indicate 'hlft tor calcium lin" Hand K. One IiQht-y.or equol. about 9.5 Irlillon spedral shiff Info its radial kilometer., or 9.5 X 1012 kilomet.r •. Di'fane .. are bo.ed on on .Ipontlon rate of 50 km/ste per million po ..ec •. velocity, to arrive at a veloc­ Ify distance l'tI/af/on. -R-ES-O-N-A-N-C-E--I--D-e-ce-m--be-r--20-0-5----------~-------------------------------7 Jayant I' Narlikar Physicists already had an explanation for such spectral shifts. First discovered by the Dutch physicist C Doppler (1803-54) in the case of sound waves, the explanation is equally applicable to light waves. The shifts are expected to occur whenever there is a relative motion between the source and the observer. If the relative motion is one of recession, a redshift is observed: if it is of approach, there will be a violetshift. The magnitude of the redshift can be used to calculate the speed of recession of the source. Edwin Hubble and his young colleague Milton Humason carefully analysed the data on spectral shifts. In particular, before deciding whether a distant nebula was approaching or receding Looking at his from our galaxy, they corrected for our own motion relative to the distance estimates, galactic centre (recall that the earth moves around the sun, which Hubble noticed a in turn moves around the galactic centre). definite pattern: the greater the Meanwhile, using other tools of imaging, astronomers had made distance, the larger enough progress to be able to make estimates of the distances of the redshift, and many of these faint nebulae. Using an (admittedly crude) assump­ hence greater the tion that a fainter object is farther away, they thus had a first speed of recession glimpse of the size of the extragalactic universe. Looking at these of the nebula. In distance estimates, Hubble noticed a definite pattern, namely, the 1929 Hubble greater the distance, the larger the redshift, implying therefore a expressed this greater speed of recession of the nebula. Hubble in 1929 expressed pattern as a this pattern as a velocity-distance relation: velocity-distance V=Hxr relation. where Vis the velocity of a nebula and r its distance. The constant of proportionality, H, in the above relation is called Hubble's constant, and the relation Hubble's law. It is clear from the above relation that 1/H has the dimension of time. Hubble's own estimate of 1/B, of about 1.8 billion years, turned out to be a gross underestimate, largely because the nebular distance estimates of the 1920's contained several large systematic errors. The present estimates of 1/H are in the range of 10 to 15 billion years. -8------------------------------~------------------------------- RESONANCE I December 2005 ~ " Origin (?) of the Universe 'X~t'<, "'--" The Expanding Universe The expanding universe exhibits The velocity-distance relation which Hubble obtained originally two important from observations of eighteen nebulae has since been determined properties: there for a much larger number of galaxies out to distances more than are no privileged several hundred times greater than those in his original sample. positions and no The phenomenon of the redshift and its relationship to distance preferred irecti ons. appears to hold. So we have prima facie evidence that other Fillure 2 The Expandlnll galaxies appear to be rushing away from our own. Have we finally Universe: An anolollY ex­ arrived, then, at a situation which singles out our location in the plalnlnll this phenomenon universe as a somewhat special one? Is that of a balloon with do" G, Gr ••• on It As the No. A little reflection on the veloci ty-distance relation will assure balloon Is blown up the do" us that there is nothing special about our location. Observers move away from one an­ other. Each dot may 'think' viewing the galactic population from another galaxy will find the that the othets ani reced­ same law as Hubble, with all galaxies rushing away from their Ing from It own. Inflate , As we inflate the balloon, the dots ~, 62 and 63 on its surface move away from each other. Yet, no dot can claim a privileged site on the spherical surface. -RE-S-O-N-A-N-C-E--I--D-ec-e-m-b-e-r-2-0-05----------~-------------------------------9 Jayant V Narlikar Weyl's postulate To appreciate this remark, imagine the universe as a lattice assumes that the structure, with galaxies occupying the lattice points. Suppose large scale motion now that the entire lattice expands. All points of the lattice will of typical units, the therefore move away from one another, with no particular point galaxies, is highly occupying a preferred position. This analogy helps us to visualize streamlined. This the oft-repeated statement that the universe is expanding. serves as a reasonable first The expanding universe exhibits two important properties: there approximation. is no privileged position, as we just mentioned, and no preferred direction. Observers brought blindfolded to any location cannot, on removing their blindfolds, tell where they are or in what direction they are looking! Perfect democracy prevails in this universe. This is the kind of symmetry that Aristotle would have liked! But how does his modern counterpart in theo­ retical cosmology deal with the question of modelling the Figure 3 The Weyl Postu­ universe? late: A streamlined motion ofgalaxiesshown In (b) may be contrasted with chaotic Mathematical Models: Basic Assumptions motion In (a).
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