Cosmology and extragalactic astronomy Mat Page Mullard Space Science Lab, UCL 3. Cosmological concepts and Olbers’ paradox

7. Cosmological concepts and Olbers’ paradox This lecture: The very basics of cosmological models Olbers’ paradox and its solution Slide 2

What is cosmology? The Science of the Universe as a whole –Chambers 20 th Century Dictionary The contents, shape, history and fate of the Universe. Slide 3

Brief History of Cosmology The origin of the Universe is something people have always been interested in. Early theories of cosmology are enshrined in mystical, religious and cultural texts. Important building blocks to modern cosmological perspective: –Greek philosophers: spherical Earth –Copernicus 1543: the removal of the earth from the centre of the Universe –Newton ( ): theory of gravitation Slide 4

–Herschel ( ): mapped the disk shape of our own galaxy and proposed that the nebulae were ‘island universes’. –Einstein: General relativity (1916), static universe –Friedmann (1922), Lemaitre (1925): expanding universe cosmology –Hubble: recession law of galaxies (1929) –Bondi, Gold and Hoyle 1948: steady state cosmology –Penzias and Wilson 1965: cosmic microwave background -----end of steady state, modern, big bang era –Alan Guth 1981: inflation –Perlmutter, Schmidt 1997: accelerating universe Slide 5

Cosmological Principles Only one Universe! –Experiments are hard. –philosophy is a little more important than is normal in science. Copernican Cosmological Principle: The Universe is homogeneous and isotropic on large scales –There is nothing special about our vantage point. –The laws of physics are the same everywhere Perfect Cosmological Principle: Universe constant in time too. –Led to steady state cosmology Slide 6

Anthropic Cosmological Principle Weak ACP: –We can only observe a Universe that is congenial to life. –basically a selection effect. –If the universe didn’t support life, we couldn’t observe it. –Allows a special epoch, and to some extent a special vantage point. Strong ACP: –The Universe is the way it is in order to produce rational carbon-based lifeforms who are capable of studying Cosmology. Slide 7

Olbers’ Paradox Why is the night sky dark? Kepler, Halley, de Cheseaux, Olbers (1823) Assume the Universe is homogeneous, isotropic and infinite –homogeneous: the same in different places –isotropic: the same in all directions i.e. the universe is full of roughly uniformly- distributed stars a bit like the Sun. Slide 8

Now imagine a thin shell surrounding the earth, of radius R and thickness << R. –We can calculate the amount of light arriving at Earth from this shell. Now consider a shell of twice the radius, and the same thickness. –The stars are on average twice as distant, so the flux from each is a quarter as large –but there are 4 times as many stars so they provide as much light as the first shell. Now sum up the light from an infinite number of shells –an infinite amount of light should be arriving! Slide 9

Resolution of Oblers’ Paradox The Universe could be filled with a tenuous absorbing medium - dust or something similar –No good. It would heat up until it emitted as much radiation as it absorbed Stars hide behind each other –they can’t be lined up if the Universe is homogeneous –at best, end up with the situation that every line of sight reaches a stellar photosphere –the night sky should have the average brightness of a stellar photosphere(!) Expansion -> redshift lowers intensity –good idea, but not enough, and the sky is not bright enough in the IR. Slide 10

The solution The Universe is only 14 billion years old. Light from stars more distant than 14 billion light years has not have reached us yet. There is a limit to the size of the Universe which can illuminate the night sky. The sky is allowed to be dark. –the numbers do work out. Cosmology isn’t broken (phew). Slide 11