How Fast Is The Universe Expanding? The Riddle Of Two Values For The Hubble Constant

In the last decades, our understanding of the Universe has made tremendous progress. Not long ago, “precision astronomy” was thought to be an oxymoron. Nowadays, satellite experiments and powerful telescopes on earth were able to measure the properties of our Universe with astonishing precision. For example, we know the age of the Universe with an uncertainty of merely 0.3%, and even though we still do not know the origin of Dark Matter or Dark Energy we have determined their abundance with a precision of better than 1%.

There is, however, one value that astronomers have difficulty in pinning down: how fast our universe is expanding. Or, more precisely, astronomers have used multiple methods of estimating the Hubble constant, and the different methods are converging quite tightly on two different values! This clearly can’t be true, but nobody has yet figured out how to reconcile the results, and further observations have only improved the precision, deepening the conflict. It’s likely that we’ll need either new astronomy or new physics to solve this puzzle.

The Discovery of the Expanding Universe

In the 1920s Edwin Hubble used the newly built telescope at Mount Wilson Observatory to study fuzzy objects known as nebulae. Back then, astronomers were arguing whether these nebulae are clouds of stars within our Milky Way or if they are whole different galaxies. Hubble discovered stars within these nebulae whose brightness slowly fades in and out. These were known as Cepheids and previously studied by Henrietta Levitt who showed that there was a tight relationship between the star’s intrinsic brightness and the period of its variation. This means Cepheids could be used as so-called standard candles which refers to objects whose absolute brightness is known. Since there is a simple relationship between how the brightness of an object decreases with distance, Hubble was able to calculate the distance of the Cepheids by comparing their apparent and intrinsic brightness. He showed that the Cepheid stars were not located within our galaxy and that nebulae are actually distant galaxies.

Hubble also measured the velocity at which these distant galaxies are moving away from us by observing the redshifts of spectral lines caused by the Doppler effect. He found that the further away the galaxy is located, the faster it is moving away from us described by a simple linear relationship.

\bf v = H_0 d

The parameter H0 is what is known as the Hubble constant. Later the Belgian priest and physicist Georges LemaĆ®tre realized that the velocity-distance relationship measured by Hubble was evidence for the expansion of the Universe. Since the expansion of space itself causes other galaxies to move away from us we are not in any privileged location but the same effect would be measured from any other place in the Universe. An effect that is sometimes illustrated by drawing points on a balloon, when it is inflated the points move away from each other at a speed that depends on their distance. It is also better not to think of the cosmological redshift as being caused by a real velocity as the parameter v in the above equation can easily exceed the speed of light. Continue reading “How Fast Is The Universe Expanding? The Riddle Of Two Values For The Hubble Constant”

3D Printed Wobbly Wheels Put Through Their Paces

When we talk about wheels, the vast majority of the time we’re talking about ho-hum cylindrical rollers as seen on all manner of human conveyances. However, there are all manner of wild and wacky shapes that roll, and having had some experience with them, [Maker’s Muse] decided to take a shot at having a robot drive on them. (Video, embedded below.)

The benefit of a 3D printer is that it makes producing these parts with strange geometries a cinch. The video shows a variety of designs, from the wobbly “Nightshades” to the entertaining “Prongle” wheels being put through a variety of tests. In an attempt to equalise the playing field, each design was matched in its surface area so as not to artificially bias the results.

While the wobbly designs look strange, they also come with some benefits over simple disc wheels, providing extra traction on both carpet and sand. Particularly impressive was the performance of the 8-spoke wheels on the beach, though as this design mimics real-world sand tyres, we’re not surprised at the results. We’ve seen similar 3D printed parts do the job for driving on water, too.

Continue reading “3D Printed Wobbly Wheels Put Through Their Paces”

To Kill A Blockchain, Add Naughty Stuff To It?

Even if not all of us are blockchain savants, we mostly have a pretty good idea of how they function as a distributed database whose integrity is maintained by an unbroken chain of conputational hashes. For cyryptocurrencies a blockchain ledger stores transaction records, but there is no reason why the same ledger can not contain almost any other form of digital content. [Bruce Schneier] writes on the potential consequences of content that is illegal or censored being written to a blockchain, and about how it might eventually form a fatal weakness for popular cryptocurrencies.

It’s prompted by the news that some botnet operators have been spotted using the Bitcoin ledger to embed command and control messages to hide the address of their control server. There have already been cases of illegal pornography being placed within blockchain ledgers, as well as leaked government data.

[Schneier] uses these two content cases to pose the question as to whether this might prove to be a vulnerability for the whole system. If a government such as China objects to a block containing censored material or a notoriously litigious commercial entity such as Disney objects to a piece of copyrighted content, they could take steps to suppress copies of the blockchain that contain those blocks. Being forced by hostile governments or litigious corporations to in effect remove a block from the chain by returning to the previous block would fork the blockchain, and as multiple forks would inevitably be made in this way it would become a threat to the whole. It’s an interesting possible scenario, and one that should certainly be ready by anyone with an interest in blockchain technologies.

Only a few weeks ago we looked at another threat to blockchain technologies – that they might be legislated out of existence by environmental rules.