Stars and Galaxies

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Slide 84

Hubble’s Evidence

Doppler shifting - wavelength emitted by something moving away from us is shifted to a lower frequency

Sound of a fire truck siren - pitch of the siren is higher as the fire truck moves towards you, and lower as it moves away from you

Visible wavelengths emitted by objects moving away from us are shifted towards the red part of the visible spectrum

The faster they move away from us, the more they are redshifted. Thus, redshift is a reasonable way to measure the speed of an object.

When we observe the redshift of galaxies, almost every galaxy appears to be moving away from us – the Universe is expanding.

Slide 85

Predictions for the Big Bang Model

The expansion of the Universe

Edwin Hubble's 1929 observation that galaxies were generally receding from us provided the first clue that the Big Bang theory might be right.

The abundance of the light elements H, He, Li

The Big Bang theory predicts that these light elements should have been fused from protons and neutrons in the first few minutes after the Big Bang.

The cosmic microwave background (CMB) radiation

The early universe should have been very hot. The cosmic microwave background radiation is the remnant heat leftover from the Big Bang.

Slide 86

Evidence for Big Bang

Red shift - as light from distant galaxies approach earth there is an increase of space between earth and the galaxy, which leads to wavelengths being stretched

In 1964, Arno Penzias and Robert Wilson, discovered a noise of extraterrestrial origin that came from all directions at once - radiation left over from the Big Bang

In June 1995, scientists detected helium in the far reaches of the universe - consistent with an important aspect of the Big Bang theory that a mixture of hydrogen (75%) and helium (25%) was created at the beginning of the universe

Slide 87

When Did the Universe Form?

~13.7 billion years ago

How do we know?

Spreading (Red Shift) - know distances, rates of retreat, relative positions

Pervasive background radiation of 2.7°C above absolute zero - afterglow of the Big Bang

http://timeline.aps.org/APS/resources/85_06a.jpg

Cosmic background radiation temperature on celestial sphere