One billion light years
One billion light years, what is this concept?
A light year is the distance that light travels in one year. Light speed! It's the acknowledged limit of speed, at 299,792,458 meters per second, able to circle the Earth seven and a half times in the blink of an eye. See how fast it is? Let it run for a whole year, and the distance it covers is called one light year. Now, everyone move your mouse to the bottom left corner of the screen, click "Start" - "Programs" - "Accessories" - "Calculator," and calculate it together. A year has 31,536,000 seconds, and light travels 299,792,458 meters per second. Multiply these numbers, and you get 9,454,254,955,488,000 meters, which is approximately ten trillion kilometers. You say what, it's just an astronomical number? Of course, astronomical numbers must be astronomical figures! But this is merely the length of one light year.
When we see stars more than a billion light years away, the beam of starlight that enters our eyes has already been traveling through the vast universe for a billion years. In other words, what we see now is merely its appearance from a billion years ago! We won't know what it looks like now until another billion years have passed... Chilling, isn't it?
It is generally believed that the universe was born about 15 billion years ago. Therefore, the diameter of the most extensive observable universe space can only possibly be within the range of 15 billion light years. The light from places 15 billion light years away, when we see it, has already traveled across the universe for 15 billion years, and that is the image of the universe at its birth!!!
The image below shows the observation of the universe on the scale of billions of light years. Each pixel point on it represents something incredibly ancient.