ASTR 200 : Lecture 27. Expansion and large scale structure

Transcription

1 ASTR 200 : Lecture 27 Expansion and large scale structure 1

2 A preference for recession In 1912, american astronomer Vesto Slipher began painstakingly acquiring spectra of `spiral nebulae' and was the first to observe the wavelength shifts of their spectral lines By 1917, he had observed 25 galaxies, and found that 21 had redshifts In 1927, Georges Lemaitre, a Belgian priest and mathematician, realized that the redshifts appeared to be proportional to the distance, and he proposed that they resulted from a homogenous expansion of the Universe. This idea later became known as the Big Bang theory after British astronomer Fred Hoyle referred to Lemaitre's theory as 'this big bang idea' on a BBC radio interview 2

3 The first `Hubble diagram' In a selection of spiral galaxies, Hubble thought he identified 'the brightest star', which he used as a standard candle (later versions would use `Cepheids'). Thinking these were about one million solar luminosities he could use their observed flux to calculate the distance to the galaxy Using this, he published the first `Hubble diagram' in 1929 (In fact, Hubble was NOT looking at brightest single stars! He was looking at compact bright star forming km/s regions which are much brighter, so he underestimated the distances to the galaxies...) Nevertheless, this was the first quantitative measurement of the universe's expansion 3

4 The first Hubble diagram A Hubble diagram shows that, once one gets past the Local Group, there is roughly linear relationship between the recessional speed (derived from the redshift z = v/c) and distance d to the Galaxy This proportionality is what Lemaitre suggested in an expanding universe v d v= H o d (Because of the distance error Hubble made, the distances on this plot are all too low by a factor of 2 3) km/s The constant H is now o called the Hubble constant. Because of the distance error, the graph shows a value Ho ~ 500 km/s /Mpc that is much too high. (The local group is so close that v 4 random speeds dominate over the expansion) The local group 1 Mpc d

5 The path to improved distances The famous astronomer Edwin Hubble was a heavy user of the new (completed 1917) powerful 100-inch telescope at Mt Wilson near Los Angeles. His colleague Humason compiled radial velocities for 620 galaxies in the 1920s Hubble estimated distances to some of these galaxies using Cepheid variables, using the relations discovered by Henrietta Leavitt that linked their period to their luminosity Leavitt (working at Harvard College Observatory) observed many variable stars, in particular a set that, like the giant star delta Cephei, were very intrinsically bright and varied with a well defined period. 5

6 The Cepheid period-luminosity relation more luminous The period of the brightness oscillation (which we now understand to be the entire star pulsating appreciably in size) is very well correlated to the intrinsic luminosity of the star. This makes Cepheids `standard candles'. If you can measure the period, you KNOW the mean luminosity, and then from its measured flux you can calculate how far away it must be 6 This was a critical advance in cosmology; Hubble said she deserved the Nobel prize for this work. An attempt to nominate her failed because she succumbed to cancer in 1921 (the Nobel prize is never awarded posthumously). `She provided the key to determine the size of the cosmos; Hubble only turned it in the lock.'

7 Modern value of the Hubble constant Measurement of the value of the Hubble `constant' (note it's not constant over the age of the universe!) consumed the rest of the 20th century. In late 1950s, Baade corrected an error in Cepheid interpretation, bringing the estimate of Ho< 200 km/s /Mpc Modern estimate via a variety of methods is : 68 +/ 1 km/s /Mpc 7

8 So, we are the center of the universe, right??? If all the galaxies are moving away from us, then we must be the center of the universe This popular misconception is wrong. Lemaitre had immediately understood that this is what every observer would see A good spatial analogy (and these kinds of arguments are all analogies) is the raisins in a loaf of baking bread. The raisins stay the same size, but as the loaf's material (the volume of space) expands, they move away from each other. Each raisin sees all the others moving away 8

9 Another analogy This is an `embedding' analogy. `Space' is confined to the 2-dimensional surface of an inflating baloon; distances are measured ONLY along the surface of the balloon. As the balloon expands, galaxies stay the same size but the space between them increases. So perpendicular to the balloon's surface is like the time dimension All galaxies see all others receding NO spatial point is the expansion's center 9

10 Universality of Hubble's Law 10

11 Universality of Hubble's Law 11

12 A rough age of the universe If galaxies are receding from each other, then in the past they were much closer to us. At what time would they have all been on top of each other? Suppose the expansion speed doesn't change with time (THIS IS NOT TRUE!). Then a galaxy that is currently at distance s and receding with speed v would have been at zero distance at a time t=s/v But v=hos, so th=1/ho and this timescale is called a `Hubble time' Evaluating this using the present value (that is, the current expansion rate) of the universe gives the estimate : 22 1 Mpc m 17 t= = s 14.7 Gyr 68 km / s 67,800 m / s In fact the velocities have NOT been constant because gravity (and other phenomena) have been affecting the expansion. Nevertheless this estimate is not far from the current estimated 13.8 Gyr `age of the universe'. Thus, the `Big Bang theory' is that the universe was in a very dense state ~14 Gyr ago and has been expanding ever since. 12

13 Could something else be going on? We do not directly observe the recessional motion of galaxies, only that all distant galaxies are red-shifted proportional to their distance. Is it conceivable that some other unknown effect causes the redshift? As an example, the so-called `tired light' hypothesis proposes that photons somehow lose energy E=hν (thus dropping to lower frequencies ν and hence higher wavelengths) as they propagate over cosmic distances. Is there any independent evidence that the universe is expanding/changing? (The alternative 'steady state' theory was that the universe has always looked the same.) Yes. Many different kinds of observations support the Big Bang theory: Radio source counts Evolution of the galaxy distribution Cosmic background radiation Cosmic ionization Cosmic nucleosynthesis 13

14 There is structure in the universe, on scales of ~100 Mpc These filamentary structures arise as gravity causes matter to clump over the age of the universe 14 Note: once redshifts z>0.1 need to use the relativistic Doppler formula

15 This kind of structure gets produced in cosmological simulations On Gpc scales the universe has the same total amount of matter in any box `homogenous' Right: computer simulation of the galaxy distribution after 14 Gyr of evolution after the Big Bang 15

16 This is called the `cosmological principle' This is a principle, not a derived 'physical law' `The distribution of matter in the universe is homogeneous and isotropic, when viewed on a large enough scale.' Homogeneous : No matter what your location is in the universe, on large scales you see the same thing. Isotropic : No matter which DIRECTION you look in the universe, you see the same thing. Eg of homogenous non-isotropic universe could be one with a magnetic field pointed in some specific direction, everywhere There is VERY strong evidence, from observations of the cosmic background radiation, that early in its history, the universe was homogeneous and isotropic to a very high degree. Only later in its history did inhomogeneities develop due to gravitational instability. It is these instabilities that lead to stars, galaxies, and planets! 16

17 Galaxy 'Peculiar Velocities' Because galaxies attract each other, the pulls create motions that are added on top of the Hubble expansion Typically km/s, creating scatter Can be ~ km/s in rich clusters 17 Once scales of ~100 Mpc are reached, the Hubble speeds completely dominate

18 `Finger of God' Effect 18 (z<0.04) In redshift surveys of the local universe, galaxy superclusters create high internal motions, and we pick up the RADIAL parts which get incorporated into the redshifts In this plot, distance from origin is observed redshift z