Question 44.2: Looking back in time. Astronomers often think of their teles......

Physics for Scientists & Engineers with Modern Physics [1000089]

Looking back in time. Astronomers often think of their telescopes as time machines, looking back toward the origin of the universe. How far back do they look?

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The distance in light-years measures how long in years the light has been traveling to reach us, so Table 44-1 tells us also how far back in time we are looking. For example, if we saw Proxima Centauri explode into a supernova today, then the event would have really occurred 4.3 years ago. The most distant galaxies emitted the light we see now roughly $10^{10}$ years ago. What we see was how they were then, $10^{10} \,\mathrm{yr}$ ago, or about $10^{9}$ years after the universe was born in the Big Bang.

TABLE 44-1 Astronomical Distances

\begin{array}{lc}\hline \rm Object & \begin{array}{c} \text{Approx. Distance} \\\text{from Earth (ly)}\end{array} \\\hline \rm Moon & \begin{array}{r}4 \times 10^{-8} \\\end{array} \\\rm Sun & \begin{array}{r}1.6\times 10^{-5} \\\end{array} \\ \begin{array}{l} \text {Size of solar system} \\\text{(distance to Pluto)}\end{array} & 6 \times 10^{-4} \\ \begin{array}{l} \text{Nearest star} \\\text{(Proxima Centauri)}\end{array} & 4.3 \\ \text{Center of our Galaxy} & 2.6 \times 10^{4} \\\text{Nearest large galaxy} & 2.4 \times 10^{6} \\ \text{Farthest galaxies }& 10^{10} \\\hline\end{array}

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