6.1

The Universe is over 13 billion years old

The estimated age of the Universe based on modern observations and on state-of-the-art cosmological models for its early evolution, is approximately 13.8 billion years. Cosmology is a research field which studies the evolution and the structure of the Universe.

6.2

The Universe is homogeneous and isotropic on the large scale

At the largest scales (approximately greater than 300 million light-years), matter in the Universe appears to be uniformly distributed. Because of this nearly uniform density and structure, the Universe looks nearly the same at any location (homogeneous) and in every direction (isotropic).

6.3

We always observe the past

Due to the finite speed of light, we never see objects as they are now, but always as they were in the past. We can only see the Sun as it was about eight minutes ago, since light from the Sun takes about eight minutes to reach us. We see the Andromeda galaxy as it was about 2.5 million years ago, since it takes the galaxy’s light that long to arrive on Earth. In this way, astronomers always observe the past, even up to 13.8 billion years ago. Observing astronomical objects at various distances thus provides us with a cross-section of cosmic history. Since on average, the Universe has the same properties everywhere, this cross-section provides valuable clues as to our own history.

6.4

We can only directly observe a fraction of the total Universe

Since light travels in space at a finite speed, there are distant regions of the Universe we cannot yet observe. The reason for this is simply that light from those regions has not had enough time to reach our detectors on Earth. We can only see objects that lie inside a certain region that is called the “Observable Universe,” comprising all objects whose light has had the necessary time to reach us. Of particular interest are very distant objects near the border of that region. Those appear to us in the form they were when the Universe had just begun.

6.5

The Universe is mainly composed of Dark Energy and Dark Matter

The stars, the air we breathe, our bodies and everything we see around us consist of atoms, which themselves are composed of protons, neutrons and electrons. This so-called baryonic matter, is what we interact with in our daily life. Observational evidence shows that it represents only about 5% of the total composition of the Universe. In fact, the Universe is mainly composed of an unknown form of energy referred to as Dark Energy (around 68%), and an unusual form of matter called Dark Matter (around 27%). The nature of the so-called Dark Energy and Dark Matter is an active area of research, especially through observations of their influence on the baryonic matter.

6.6

The Universe is expanding at an accelerated rate

Observational evidence shows that the Universe is expanding at an accelerated rate, which is attributed to Dark Energy. As the Universe expands in a systematic way on large scales, clusters of galaxies move away from each other. In the modern models, all distances between galaxy clusters grow in proportion to the same universal scale factor. Observational data show that the more distant a galaxy is from us, the faster it moves away from us (Hubble-Lemaître Law). Hypothetical alien observers in other galaxies would find the same. Bound systems, such as clusters of galaxies, and groups of galaxies bound by their own gravity, or galaxies themselves, are not affected by cosmic expansion. Within galaxy clusters and groups, individual galaxies can be orbiting each other, or they can be on a collision course with each other. The latter is true for the Milky Way Galaxy and the Andromeda galaxy.

6.7

The expansion of space causes light from distant galaxies to be redshifted

Cosmic expansion influences the properties of light in the Universe. Light reaching us from distant galaxies is increasingly redshifted with larger distances. This cosmological redshift can be understood directly in terms of wavelengths of light increasing (stretching to longer wavelengths) with the cosmic scale factor. That is why distant galaxies can only be observed in the infrared or radio bands, and why the Cosmic Microwave Radiation reaches us mostly in the microwave regime.

6.8

The natural laws (e.g. gravity) that we study on Earth appear to work the same way throughout the Universe

There have been many tests to see if the laws of physics, such as the laws governing gravity, thermodynamics and electromagnetism, are the same on Earth and in the distant Universe. So far, all such tests indicate that the fundamental laws of physics apply throughout the whole Universe.

6.9

The large-scale structure of the Universe is composed of filaments, sheets and voids

Large redshift surveys of the Universe have revealed that on large scales in the order of a few hundred million light-years, the Universe resembles a three-dimensional, sponge-like web of filaments and voids, which astronomers call the “cosmic web”. Filaments and sheets contain millions of galaxies. These large-scale structures extend over hundreds of millions of light-years and are typically tens of millions of light-years thick. The filaments and sheets form boundaries around voids, which are in the order of hundred million light-years in diameter, and contain only very few galaxies.

The oldest electromagnetic radiation, emanating from the most distant regions in the Universe that we can observe, is the Cosmic Microwave Background radiation. It is the relic left over from the hot and dense early Universe, imprinted with information from a time when the Universe was about 380,000 years old. The Cosmic Microwave Background allows us to measure key characteristics of the Universe as a whole: the amount of Dark Matter, baryonic matter and Dark Energy it contains, the geometry of the Universe and its current expansion rate. The Cosmic Microwave Background shows that the Universe is nearly isotropic and thus also provides indirect evidence for homogeneity.