How Polish Astronomers are Measuring the Universe
Most people's everyday concerns do not reach out beyond our planet's atmosphere, nor do they have anything to do with notions like the observable universe, space-time or eclipsing binaries.
However, these are everyday matters to astronomers, and Polish Professor Grzegorz Pietrzyński is undoubtedly an outstanding representative of this strange star-gazing breed. Along with a team of international scientists, he created a scientific project called Araucaria. Their aim is to develop more accurate ways of measuring distances in the Universe, and they have been very successful so far. Last year Professor Pietrzyński received an award from the Polish Ministry of Education for his work on measuring the distance to the Large Magellanic Cloud.
But first, let us begin with the simplest of questions: how do we measure distance? Here on earth, there are plenty of measuring devices to choose from. Most of us are familiar with the simple mechanical types, like a ruler, a tape measure or an odometer in a car. When it comes to measuring greater distances, there are more sophisticated techniques. The distance to the Moon, for instance, has been very accurately measured with a laser beam. Humans have planted reflective mirrors on its surface, and the beam is fired at those mirrors, and then reflected back to the Earth. We measure the time the beam took to go to the Moon and back, and knowing the speed of light, we can calculate the distance it covered. But on a cosmological scale, the 380,000 kilometres that separate us from our satellite are next to nothing. To give you an idea, the laser beam reaches the Moon in a second or so, thus we can say that the Moon is a light second away from us. A light year then, is the distance the beam would have covered travelling at the speed of light for a whole year. Keeping that in mind, try to wrap your mind around the fact that the Andromeda Galaxy (our cosmological next-door neighbour) is about 2.5 million light years away. But surely we couldn't have waited for 5 million years for a beam of laser light to go there and back, so how do we even know that?
Determining these intergalactic distances is a step by step process. First we need an anchor point - precise measurement for one object, and once it's pinned down, we use it as a point of reference for other objects. It's important to choose this anchor point wisely - it will determine the accuracy of any further distance measurement we'll make. Astronomers have chosen the Large Magellanic Cloud, our neighbouring dwarf galaxy. So ultimately the quest for improving our knowledge of the distances in the cosmos is really a quest for finding out exactly how far away the Large Magellanic Cloud is. This is where Professor Pietrzyński and his team step in. They have found some binary stars (two stars orbiting one another) in the LMC, and then started long-term observations. What was precious about these particular binaries,was that they were eclipsing. This signifies that, from where we stand, one of the stars obscures the other every now and then, much like the Moon obscures the Sun during an eclipse.
After eight years of careful observations and analysis of these eclipses, they were able to determine the size of these stars, as well as their angular size, a measure of how big they appear in our field of view. The distance was easily calculated using those two parameters. Of course it was not the first such measurement, many others have been performed before, but Professor Pietrzyński’s research has yielded the most likely result so far - accurate to 2.2 percent. An incremental increase in accuracy may not seem very significant, it is worth emphasising that it improved our estimate of the distances to all other galaxies, and hence gave us a better idea of the size of the observable universe. However the real motive behind this scientific achievement lies elsewhere. Knowing the distances is just a means to an end.
What we are really after is a better estimate on the Hubble’s constant, a very important parameter in modern cosmology: it determines our understanding of how the Universe has been evolving, and what will happen to it in the future. The constant, an estimated value of the expansion rate of the Universe, was named after Edwin Hubble, an American astronomer, who made an important discovery at the beginning of the 20th century. Looking at the distant galaxies he noticed that the further away an object is, the faster it seems to be moving away from us. The idea is easier to grasp if you imagine a dotted surface on a balloon being filled with air. The distances between the dots grow faster and faster as the balloon expands. That discovery is the cornerstone of modern cosmology. First of all it proves the expanding nature of the Universe, and secondly it shows that it must have had a beginning. Imagine the Universe as billions of galaxies peppered across space, moving away from one another. Now imagine you run the clock backwards, and what you get is a massive collapse. If you run it back far enough, all of those galaxies end up condensed in one point in space. That is where and when it all started and that moment is called the Big Bang nowadays. So, as you see, Hubble's constant is not only crucial in determining the size of the Universe and its rate of expansion, but also its age. Thanks to Professor Pietrzyński and his colleagues, we understand the mind boggling cosmos out there a little bit better.
Authors: Antoni Mitraszewski and Agnieszka Mitraszewska, July 2015