Here’s an easy viewing challenge for this week. Find a giant planet by looking high in the sky as soon as it gets dark. The brightest “star” you see is Jupiter. Download the sky map and see how many constellations and stars you can identify. https://skymaps.com/skymaps/tesmn2401.pdf
Now let’s continue the story of how Edwin Hubble kept expanding our universe. He didn’t quit looking for more distant galaxies after he found a “standard candle” cepheid variable star in the nearby Andromeda galaxy in 1923. (Remember, standard candles are objects whose actual luminosity is known, so they can be used to determine distances.) He found Cepheids in more galaxies and gaged their distances. Most of the galaxies nearby seemed to move randomly. Equal numbers were approaching as are receding from us. (Remember: changes in an objects light spectrum can tell if it is coming toward us or going away. This is called its Doppler Shift.)
Something odd began to reveal itself as Hubble and his assistant Milton Humason measured the distance to very distant galaxies. They realized the more distant a galaxy was, the faster it was receding from us. By 1929 they had enough data to publish what became known as “Hubble’s Law”. V=HoD. (Recession velocity varies directly with distance.) All this means is that the more distant an object is, the faster it is moving away from us. Twice the recession velocity (which is easy to measure), means the object’s distance is doubled. (A new way to measure distance.)
Research slowed during World War II, but when the giant 200-inch Palomar telescope came on line in 1948, Hubble’s “Law” was confirmed. The value of “Hubble’s Constant”, Ho, was measured to about 10% accuracy. For every additional million parsecs of distance, a galaxy would be moving away from us by an extra 70 kilometers per second. One reason for launching the Hubble Space Telescope in 1990 was to measure Hubble’s constant more accurately.
So, why is this important? Think of what increasing expansion rate with distance implies. If you imagine moving backwards in time, distant objects are returning to an earlier position faster than nearer objects. At some point in the past everything would have been at the same place! For an example, look at the balloon analogy (see image). On the right, the balloon (the universe) is expanding. Moving to the left is moving back in time and the balloon (the universe) gets smaller. There must have been a time that the balloon shrinks to zero, and the universe that the balloon represents, began! This idea was called “The Big Bang”. If you examine the dimensions of Hubble’s constant, you find that its reciprocal, 1/Ho comes out as time. Plugging in the numbers, you can calculate that at a time about 14 billion years ago, all the matter and energy of the universe was in one spot. It was incredibly hot and dense, and it erupted to become the universe we see today. Amazing what good measuring and a little math can unveil.
Is this incredible tale true? Is it the final word? Well, no. Not quite yet. 1998 brings a new twist.
Questions or comments: James Hill, NASA/JPL Solar System Ambassador, jhill6333@gmail.com
