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Name: Glenn
Status: other
Grade: n/a
Country: New Zealand
Date: N/A 


Question:
It seems settled that the universe is about 13.7 billion years old. We are told that Hubble can see back in time/distance about the same period of time, by looking at galaxies that are about 13.7 billion light years away from us. (Ignoring a small time period after the big bang but before galaxies were formed. This difference is not important to the question.) After the big bang, it's assumed that the material that eventually formed our solar system, moved outwards at a small fraction of the speed of light. 13.7 billion years later, we arrive at some point which we can call "here". It is nowhere near as far away from the origin as 13.7 billion light-years. Neither is any other matter. We look in a direction with Hubble and see light that we say departed a galaxy about 13.7 billion years ago. That galaxy was in the same vicinity as the big bang, that long ago. Its light moved away from it at the speed of light. The light that left it 13.7 billion years ago must surely be well and truly beyond "here". If we moved away from the big bang at (for argument's sake) 1% of the speed of light, yet we can still see light departing that galaxy, and if that galaxy moved in some direction other than the one we moved in, then the light we see must've left it well over 13.7 billion years ago. Paradox because we measure it as 13.7 billion light-years away. Phrasing my question in different terms, I could say that we are told scientists can detect the "afterglow" from the big bang. The photons that emanated from the big bang, must surely be much further from the origin than we are. Unless it moved in a different direction than we did and bounced off something and started heading back towards us, how the hell can we see it? Surely it's inevitably long gone from our perspective. Thus, to distill the question down: how big/old is the universe really and how do we measure it and resolve the paradox our observations disclose?



Replies:
Hi Glenn,

I think you have a misconception embedded in your question. The age of the Universe is not based on a (time=distance/speed-of-light) calculation. The age is calculated using measurements of the _expansion_ of the universe. "Expansion" of the universe is different than "two objects moving apart" -- space itself is expanding!

To illustrate the difference, consider the example of a balloon. Inflate the balloon, and draw two dots with a marker on the balloon surface. The ink dries and the two dots are fixed in place on the balloon surface, and you can measure how far apart they are. However, what if you added some more air to the balloon? The balloon would expand, and the dots would become farther apart. Yet, the dots are clearly fixed in place on the balloon surface (the marker ink did not move!). How did the dots get farther apart? The balloon expanded even though the dots did not move. Similarly, if you let out some air from the balloon, the dots will appear to come closer together.

The balloon is a good analogy for the universe expanding or contracting. Obviously the balloon is a two-dimensional surface, and the universe is three-dimensional. Also, the marker dots can not move, while planets and stars and galaxies are moving. With the balloon, it is easy to measure how much the balloon expanded, but with the universe it is much harder. Scientists use a number of measurements So the calculation of the expansion of the universe is a little more challenging.

The "age of the universe" is actually an estimate based on a range of possible ages based on a large number of measurements made using different techniques to measure the expansion of space. For example, the WMAP (a different space probe than the Hubble) has been used to measure background radiation. The radiation values are used to estimate the age of the universe.

Depending on which measurement you use, the age you calculate can be different. All measurements point to a universe that is many billions of years old, but they do not all point to exactly the same value.

Hope this helps,
Burr Zimmerman


You raise issues that have not yet been explained. First, you cannot talk about the origin of the "Big Bang" as here or there, because we have no frame of reference to measure "here" or "there". I know that sounds weird, but that is how it is. We measure distance by the increase in wavelength of various emission lines of spectra and/or other electromagnetic radiation, but that is a relative "shift".

Other not-well-understood observations also occur -- "dark matter" and "dark energy". Measurements of the force of gravity, and other measurements suggest that the stars and galaxies that we observe only account for a few percent of the amount of matter that must be present to explain the matter we observe. No one knows why!

So your question distills down to: "We do not know." Stay tuned!

Vince Calder



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