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A Cosmological Experiment

April 28, 2021

This is an experiment that you can do yourself to learn a bit about how scientists think the universe became as large as it is. All you will need is a balloon and a marker. Alternatively, you can treat it as a thought experiment!

The first step is to take your balloon and blow it up just so it holds a circular shape and then block the air off. This balloon is going to represent spacetime. Spacetime is what matter resides on. Using a marker, draw some dots on your balloon. This is our matter. 

All together, this balloon and the dots are going to represent the very early universe. When the universe was very young, it was still condensed very close together. Matter was moving away from each other, but it could not travel faster than the speed of light. What this would imply to our universe today is that the amount of the universe we can see, is all there is to see. But scientists don’t believe that this is true. Instead, they think spacetime shows us a different way of getting a large universe. 

Partially blown up balloon vs fully blown up balloon.

Take note of the distance between the dots on your balloon. Maybe pick out two favorites to measure specifically, then blow up your balloon as much as you can. Observe the distances between your dots now. They are much farther away than they were before, right? Take a close look at the dots though, they have not moved from their original position on the balloon. The balloon itself has expanded, but the dots stayed in one place. This shows your matter’s position on spacetime has not moved, but rather spacetime itself has expanded. 

This stretching of the balloon that leaves the dots in the same position is a simple way to visualize what cosmologists call the “Inflationary Theory”. This is a theory for how the universe was formed. It says that after the initial Big Bang at the beginning of time, there was a very short amount of time in which spacetime inflated to be much much larger than it was to start. Since spacetime is not matter, it can move faster than light without breaking any laws of physics. This made the matter end up far from other matter, but still technically in the same position on spacetime. Think of spacetime as a coordinate grid and matter is on it. This inflation caused the space between the grid lines to get larger, but the matter didn’t move from its position on the grid. 

The expansion of spacetime modeled as a coordinate grid.

What this means for us now ultimately is that the universe is much larger than what we can see on Earth. This is why we differentiate between the observable universe and the entire universe. The observable universe has a radius that correlates with the age of the universe. This is because light has been traveling constantly since the beginning of time so the farthest light we can see took the entire universe’s lifetime to reach us. Outside of the observable universe though, there is more of the universe that we can’t see, caused by this early inflation of spacetime when the universe was created. 

Sowell, James. (2021). Early Universe [Powerpoint Slides]. Retrieved from Georgia Institute of Technology PHYS 2022 Canvas site.

Tsujikawa, S. (2003). Introductory review of cosmic inflation. arXiv preprint hep-ph/0304257.

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About Author

Alyssa is a first year physics major at Georgia Tech. She is planning to go into astrophysics or astronomy and loves giving tours of the night sky. She also enjoys horseback riding, painting, and playing board games.

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  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.