Absorption lines of a galaxy shift toward the blue end of the spectrum when it moves toward Earth. The lines shift to the red end of the spectrum when a galaxy moves away from Earth.

Hubble’s Law How do astronomers know that the universe is expanding? The observed red shift in the spectra of galaxies shows that the universe is expanding.

Hubble’s Law In the mid-1920s, Edwin Hubble discovered that the light from most galaxies undergoes a red shift—that is, their light is shifted toward the red wavelengths. This red shift showed that nearly all galaxies are getting farther away from Earth.

Hubble’s Law Hubble also found that more-distant galaxies have greater red shifts. This relationship, called Hubble’s Law, says that the speed at which a galaxy is moving away is proportional to its distance from us. The most distant observed galaxies are moving away at more than 90 percent of the speed of light!

The Big Bang Theory What is the big bang theory, and what evidence supports it? Astronomers theorize that the universe came into being at a single moment, in an event called the big bang. The existence of cosmic microwave background radiation and the red shift in the spectra of distant galaxies strongly support the big bang theory.

The Big Bang Theory The motion of galaxies indicates that the universe is expanding uniformly. The big bang theory states that the universe began in an instant, billions of years ago, in an enormous explosion.

The universe began with the big bang 13.7 billion years ago. The Big Bang Theory Big bang occurred 13.7 billion years ago. The universe began with the big bang 13.7 billion years ago. The first stars and galaxies formed 200 million years later. The solar system, and Earth, formed about 9 billion years after the big bang. First stars and galaxies formed 200 million years after big bang. Solar system formed 4.6 billion years ago. Earth today

Evidence for the Theory The Big Bang Theory Evidence for the Theory In 1965, Arno Penzias and Robert Wilson, using a radio telescope, noticed a faint distant glow in every direction. Today this glow is called the cosmic microwave background radiation. This glow is energy produced during the big bang, still traveling throughout the universe.

Continued Expansion How can dark matter be detected? Dark matter cannot be seen directly, but its presence can be detected by observing its gravitational effects on visible matter.

Continued Expansion Much of the matter in the universe can’t be seen by astronomers. Dark matter is matter that does not give off radiation. Galaxies like ours may contain as much as ten times more dark matter than visible matter.

According to Hubble’s Law, Assessment Questions According to Hubble’s Law, the apparent wavelength of light is dependent on relative motion compared to the observer. the amount of matter in a large region of the universe is constant over long periods of time. the age of a star is proportional to the red shift of its spectrum. galaxies are moving away from Earth at speeds proportional to their distance from Earth.

According to Hubble’s Law, Assessment Questions According to Hubble’s Law, the apparent wavelength of light is dependent on relative motion compared to the observer. the amount of matter in a large region of the universe is constant over long periods of time. the age of a star is proportional to the red shift of its spectrum. galaxies are moving away from Earth at speeds proportional to their distance from Earth. ANS: D

How is dark matter in the universe detected? Assessment Questions How is dark matter in the universe detected? direct observation its effect on electromagnetic interactions its gravitational effects on matter very faint background radiation in the universe

How is dark matter in the universe detected? Assessment Questions How is dark matter in the universe detected? direct observation its effect on electromagnetic interactions its gravitational effects on matter very faint background radiation in the universe ANS: C

Assessment Questions Based on the size of the universe and its rate of expansion, the big bang is calculated to have occurred between 13 and 14 billion years ago. True False

Assessment Questions Based on the size of the universe and its rate of expansion, the big bang is calculated to have occurred between 13 and 14 billion years ago. True False ANS: T