The impact of ripples in space

When galaxies collide

Imagine a still pond on a summer afternoon. Fish rise to capture insects on the surface, making expanding rings of ripples. Insects touching the water surface and taking off again make more ripples. Then there are the ripples caused by reeds moving in the breeze. The result is the pond is filled with patterns of ripples that describe all the events taking place in the water.

The universe is like the pond, filled with ripples caused by everything taking place in it, therefore they have stories to tell, which we are learning to decipher.

When we walk across a room, or a cat jumps on a sofa, ripples (or gravity waves) are launched in the fabric of space-time. However, these will be extremely weak, and undetectable using any techniques we currently have available. On the other hand, exploding stars and colliding black holes produce extremely strong gravity waves, detectable millions or billions of light years away. They can show us things that are otherwise invisible, such as collisions between supermassive black holes in the cores of merging galaxies.

Our nearest neighbour spiral galaxy is the Andromeda Galaxy, which lies some 2.5 million light years away and is speeding directly at us at 110 km/s. As it gets closer, the two galaxies will pull at each other more strongly and the speed will increase. It will crash into us in about 4.5 billion years.

"Galactic collision" sounds like a title for the ultimate disaster movie. However, the stars are so far apart that stars hitting each other in the collision will be extremely unlikely. There will be some interaction between the dust and gas clouds filling the space between the stars, which will probably trigger the birth of many new stars and planets.

For our remote descendants, if any, the main thing will be a change in the appearance of the Milky Way in the sky, as the stars and clouds swirl and gradually coalesce into a new, larger, galaxy. This is how galaxies grow: by colliding with one another and merging. However, the merging of the central black holes will be more dramatic.

Large, spiral galaxies, like the Andromeda Galaxy, and ours have black holes in their centres with masses of a few million times the mass of the Sun. As the two galaxies merge, these black holes will spiral into each other finally merging into a single black hole. Such collisions are taking place across the universe, as galaxies grow by cannibalizing one another. Even though these events are usually too far away to see, we can detect the gravity waves they produce. One way is by means of the gravity wave telescopes we have built. However, there is a second way to observe them, by making use of radio telescopes

Scattered around our galaxy are billions of neutron stars, the rapidly rotating cores of exploded stars. These have masses greater than the mass of the Sun, compressed down to a diameter of a few kilometres. These are rotating rapidly, and with the enormous amount of inertia, their rotating rates are extremely stable.

These give off beams of radio waves, like the light beams from a lighthouse. If one of those beams points in our direction, our radio telescopes detect precisely timed pulses. We call sources of the pulses we detect pulsars; so far we have found over 2000 of these. Most of the time we see chains of extremely precisely timed pulses. However, if gravity waves cross the line of sight to a pulsar we are observing, the stretching and contraction of the fabric of space-time cause jittering in the time interval between the pulses.

We now have ground-based observatories dedicated solely to the observation of gravity waves, but having a means to use currently-available instruments - radio telescopes - to map the movement of gravity waves across space is a tremendous bonus.


Venus and Mars lie close together very low in the west after sunset. Saturn rises in the late evening and Jupiter around midnight. The Moon will be New on the 17th.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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