How to set references to 'sentence case'?

Question

Although my entries in bibliography.bib are all sentence cased, my references become Title Cased (red underlines). How could I solve this?

Edit: @user2554330's answer indicates that the *.csl file has to be edited. Since rmarkdown uses Chicago author-date format for citations and references by default it would be helpful to know where exactly in the *.csl the 'text case' to be edited can be found.

Rmarkdown code:

---
title: 'Untitled'
output: pdf_document
bibliography: "bibliography.bib"
---

Lorem ipsum dolor sit amet [@einstein_ist_1905; @hawking_thermodynamics_1983].

# References 
<div id="refs"></div>

Content of bibliography.bib:

@article{einstein_ist_1905,
    title = {Ist die {Trägheit} eines {Körpers} von seinem {Energieinhalt} abhängig?},
    volume = {323},
    url = {https://s3.amazonaws.com/objects.readcube.com/articles/downloaded/wiley/a56a92baf12b80889d9de6f28f51f22f8bec1a2b366de4e8171f0d47e890d37a.pdf?response-content-disposition=attachment%3B%20filename%3D%22Einstein-1905-Annalen_der_Physik.pdf%22&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIS5LBPCM5JPOCDGQ%2F20170830%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20170830T124037Z&X-Amz-Expires=127162&X-Amz-SignedHeaders=host&X-Amz-Signature=3de93a7b9a7edbcc0676232820c448641df14f4a22d339ad07a8cc06200d7632},
    doi = {doi:10.1002/andp.19053231314},
    urldate = {2017-08-30},
    journal = {Ann. Phys.},
    author = {Einstein, Albert},
    year = {1905},
    pages = {639--641},
    file = {a56a92baf12b80889d9de6f28f51f22f8bec1a2b366de4e8171f0d47e890d37a.pdf:C\:\\Users\\jay\\Zotero\\storage\\TTPHSMIX\\a56a92baf12b80889d9de6f28f51f22f8bec1a2b366de4e8171f0d47e890d37a.pdf:application/pdf}
}

@article{hawking_thermodynamics_1983,
    title = {Thermodynamics of black holes in anti-de {Sitter} space},
    volume = {87},
    issn = {0010-3616, 1432-0916},
    url = {http://link.springer.com/10.1007/BF01208266},
    doi = {10.1007/BF01208266},
    language = {en},
    number = {4},
    urldate = {2017-10-13},
    journal = {Communications in Mathematical Physics},
    author = {Hawking, S. W. and Page, Don N.},
    month = dec,
    year = {1983},
    pages = {577--588},
    file = {57cae4b908ae3ac722b1eaa1.pdf:C\:\\Users\\jay\\Zotero\\storage\\DZ38DDCK\\57cae4b908ae3ac722b1eaa1.pdf:application/pdf}
}

Session Info

> sessionInfo()
R version 3.5.2 (2018-12-20)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 7 x64 (build 7601) Service Pack 1

Matrix products: default

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252   
[3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C                          
[5] LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
 [1] compiler_3.5.2  backports_1.1.3 rprojroot_1.3-2 htmltools_0.3.6 tools_3.5.2    
 [6] yaml_2.2.0      Rcpp_1.0.0      rmarkdown_1.8   knitr_1.21      xfun_0.4       
[11] digest_0.6.18   evaluate_0.12  

> rmarkdown::pandoc_version()
[1] ‘1.19.2.1’

# MiKTeX 2.9

Answer 1

You can specify the style you want using the csl field in the YAML header, for example

---
title: 'Untitled'
output: pdf_document
bibliography: "bibliography.bib"
csl: the-astronomical-journal.csl
---

A big collection of styles is browsable at http://zotero.org/styles, and more details of bibliography handling are at https://rmarkdown.rstudio.com/authoring_bibliographies_and_citations.html.

Edited to add: If you have a csl file that is fine except for capitalization, then just delete lines saying things like text-case="capitalize-first", text-case="title", and text-case="lowercase" on the fields with the bad case handling, and whatever capitalization you use in your bib file will be copied into the bibliograpy.

One further edit: If you are aiming for PDF output, you can use natbib or biblatex to produce your bibliography, and use any BibTeX style that works with those. You can even use the command line command latex makebst to make your own custom style. If you've chosen to use style foo.bst with natbib, your YAML should look like

---
title: 'Untitled'
output: 
  pdf_document:
    citation_package: natbib
bibliography: "bibliography.bib"
biblio-style: foo
---

This has the advantage over the csl styles that it will respect the capitalization markup in your bib file, while allowing changes to suit your taste.