In two weeks, 2016 will end, and it seems that the solar system, galaxy and universe have not changed at all after one year. We completed another orbit around the Sun, but there were more than 4.5 billion. And although we may have noticed some major events that occur in the Universe every year, namely:
- the arrival of comets, - the beauty of meteor shower, - twinkling of the nearest star, - destructive supernovae, - these are just the most obvious changes.
Typically, we think of a year as a fairly long period of time. In human terms, a lot can happen in 365 days (or so). But compared to the universe, which is 13.8 billion years old, a year is literally an instant. Seriously, if we were comparing the age of the universe to one year, it would be comparing human life to 0.2 seconds. And even in such a short period of time as a year, subtle changes are taking place in our solar system, our galaxy and the universe, which add up to large, slow changes on the greatest time scales.
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The Earth's rotation has slowed down
Of course, you hardly noticed it. The time it takes for the Earth to rotate around its axis once - a day - is 14 nanoseconds longer than it took for such a rotation a year ago. But if you wait long enough, it increases. In four billion years, our rotation will slow down enough to eliminate leap years: there will be exactly 365 days in a year. It also follows from this that at the dawn of the solar system, the day on Earth was shorter: the Earth made a revolution in 6-8 hours, since the year consisted of more than a thousand days. But slow rotation is just the beginning.
The moon is further away this year than last
Again, you are unlikely to notice this, but there is a fundamental conservation law that makes this necessary: the conservation of angular momentum. Imagine the Earth-Moon system: they revolve around their axes, while the Moon revolves around the Earth. If the Earth's rotation is slowing down, this means that something needs to be balanced against this loss. This something is the Moon revolving around the Earth: the Moon is moving away to preserve the system. For a year, of course, you will not notice this removal, even with the help of an ingenious laser - the difference in the Moon's orbit appears in centimeters per year. But over time, when 650 million years have passed, there will no longer be such a thing as a total solar eclipse, as the Moon will be far enough away that even perfectly aligned solar eclipses are annular at best.
The sun is hotter than it was a year ago
But only on average, please note, since the variations in the Sun are even greater than the overall warming effect. They definitely can't add to the overall warming the Earth is experiencing, as the Sun's luminosity increases by about five billionth of a percent for a year, 0.0000000005%. Enough time will pass and it will become noticeable. You see, the Sun converts matter into energy, losing about 1017 kg of mass per year according to Einstein's formula E = mc2. By burning off fuel, the Sun gets hotter, starts burning fuel faster, and this leads to an overall increase in energy output. In two billion years, the sun will be hot enough to boil Earth's oceans and end the life we know. Ultimately, global warming caused by the sun will end all of us.
And all this is only in our solar system; the galaxy and everything beyond it, too, has changed over the year.
A new star was born in our galaxy, slightly smaller than the sun
In the Milky Way, new stars form on a permanent basis in nebulae, leading to clusters of young stars. Our current rate of star formation is - to the best of our modest knowledge - 0.68 solar masses per year in our galaxy. This, of course, is on average: in a hundred years, one star of 100 solar masses can form, or five tiny stars in one year. In reality, the formation of stars occurs gradually and takes millions of years. But on average, we have a new star, slightly less massive than the Sun, every year.
We've added a few chances to a supernova explosion in our galaxy
We used to think that supernovae are very rare events, because the last thing we saw was the supernova Tycho in 1572 and two Keplers in 1604, which astronomers saw with the naked eye. But since then, we've found others that have successively exploded in our galaxy, including Cassiopeia in the late 1600s and Sagittarius in the late 1800s. It is now known from observations of other galaxies that our galaxy should contain four times as many types of Ia supernovae and that we expect two to seven supernovae each century. However, this has not been fully approved. The percentage of expectation can be much higher, and even if we do not see all the supernovae, there is a possibility that they were, and one even last year. The chances are higher every year.
And on the scale of the Universe …
The universe is colder this year than last
The afterglow of the Big Bang is terribly cold: only 2.725 K above absolute zero. And yet, this temperature developed only after 13.8 billion years of cooling; before that, it was high enough to ionize atoms, rupture nuclei, even did not allow quarks and glues to form separate protons and neutrons. This cooling and expansion will continue until it reaches absolute zero. For a year we will hardly notice the difference, but the water wears away the stone. A few dozen more ages of the Universe - and we will no longer know that the cosmic microwave background ever existed.
20,000 stars have become unattainable for us
Dark energy continues to gain power and expand the universe, accelerating the scattering of distant galaxies. In about 15 billion light-years, these galaxies are moving away from us faster than the light emitted by us can travel. Of all the observed galaxies in the Universe, 97% have become lost to us forever. But the remaining 3% do not just huddle close by, they also run away faster and faster. With each passing year, the 20,000 new stars that were reachable (when traveling at the speed of light) have become unreachable. The longer we delay travel to the stars, the less we have left to visit.
The life of the universe may be long and the year may be short in the grand scheme of things, but nevertheless everything flows, everything changes. If we look close enough and accurately enough, we too will feel the passage of time. Not only here, in our home world, but also in the solar system, galaxy, the Universe somewhere out there.
ILYA KHEL