The Structure of the Universe

Finding RR Lyrae Stars Globular clusters contain many RR Lyrae stars. They are easy to find because their luminosities change over time. Since we know the absolute luminosities of RR Lyrae stars, we can now measure the distance to this RR Lyrae star (and hence the cluster) with the inverse square law of light.

The Distances to Galaxies Most galaxies are too far away for telescopes to detect RR Lyrae stars within them, so we need a brighter standard candle to measure distances to galaxies. RR Lyrae stars are not the only stars that pulsate. Cepheid stars also pulsate in a regular fashion. Because Cepheids are brighter than RR Lyrae stars, they can be detected at larger distances, even in other galaxies.

The Cepheid Period-Luminosity Relation Unlike RR Lyraes, Cepheids have a wide range of luminosities. But brighter Cepheids have longer pulsation periods, so if we measure how fast a Cepheid pulsates, we can deduce its luminosity from the relation shown below.

The Distance Ladder Using Cepheids as a standard candle, we can measure distances to galaxies out to 75 million light years (Mly). If we want to measure distances beyond this limit, we need a standard candle that is even brighter. 75 Mly 30,000 ly 1000 ly

The Tully-Fisher Relation The rotation speed of a galaxy depends on its mass (as we found when discussing dark matter). Galaxies with higher masses generally have higher luminosities. So the luminosities and rotation speeds of galaxies are correlated, which is know as the Tully-Fisher relation. So using the Tully-Fisher relation, we can estimate a galaxy’s luminosity by measuring its rotation speed, and then use that luminosity to estimate the distance of the galaxy.

Type Ia Supernovae If a white dwarf gains enough matter from another star so that its mass exceeds 1.4 M, it experiences a Type Ia supernova, which is bright enough to be seen in distant parts of the universe. These supernova always have the same luminosities, and therefore can be used as standard candles for measuring distances to the most distant galaxies.

The Distance Ladder 3 billion ly 600 Mly 75 Mly 30,000 ly 1000 ly

Mapping Galaxies in the Universe Galaxies are not scattered randomly in space. Most galaxies are in groups and clusters. These clusters are arranged in walls and filaments that are separated by huge voids. 600 (Mly) 300

Mapping Galaxies in the Universe

The Origin of Structure in the Universe Stars form from the gravitational collapse of ripples and clumps in gas and dust clouds. Galaxy clusters formed in a similar manner after the Big Bang, but on a far larger scale. 100 Mly Age of universe = 0.2 billion years

The Origin of Structure in the Universe Because it makes up most of the mass in the universe, dark matter played a crucial role in this process. Clumps of dark matter acted as the seeds that led to the formation of galaxies. 100 Mly Age of universe = 1.0 billion years

The Origin of Structure in the Universe The formation of galaxies probably would not have occurredwithout the presence of dark matter. 100 Mly Age of universe = 4.7 billion years

The Velocities of Galaxies In 1912, VestoSlipher obtained spectra of galaxies, which he used to measure their velocities through the Doppler shift of absorption lines.

Moving Toward Us (blueshifted) The Velocities of Galaxies Moving Away From Us (redshifted)

The Hubble Law Edwin Hubble measured distances to Slipher’s galaxies. He found that galaxies with higher velocities away from us (higher redshifts) have larger distances from us. This correlation between distance and velocity is known as the Hubble Law. But it’s very unlikely that we are at the center of the universe, so why are all of the galaxies moving away from us? 1.5 3 (billion light years)

The Expanding Universe At any location in a universe that is expanding, galaxies that are farther away will appear to be moving away faster. In other words, the Hubble Law would be observed by everyone in an expanding universe. Rather than indicating that we are at the center of the universe, the Hubble Law tells us that the universe is expanding.

The Cosmological Constant Einstein believed that the universe was static and unchanging. However, gravity should eventually cause a static universe to collapse. He didn’t like this idea, so he proposed the existence of anti-gravity, which he called the Cosmological Constant, that would prevent the universe from collapsing.

The Cosmological Constant Shortly after Einstein developed the Cosmological Constant, Hubble discovered that the universe isn’t static, and instead is expanding. So Einstein was wrong to assume the universe is static, and it wasn’t necessary invent anti-gravity to maintain a static universe. He referred to the Cosmological Constant has his “greatest blunder”. 1.5 3 (billion light years)

The Structure of the Universe